Performance/RedundantMerge raises error on certain double splat case

[mmedal]

Appears to be a regression related to an earlier issue: #4366. When running merge!, passing a kwsplat as the sole argument causes rubocop to raise undefined method 'source' for nil:NilClass.

---

Expected behavior

Rubocop should not raise an error.

Actual behavior

Error raised in RuboCop::Cop::Performance::RedundantMerge#to_assignments.

An error occurred while Performance/RedundantMerge cop was inspecting  test:55:8.
  undefined method `source' for nil:NilClass
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:99:in `block in to_assignments'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:92:in `map'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:92:in `to_assignments'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:47:in `block in message'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:19:in `redundant_merge_candidate'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:46:in `message'
rubocop-0.60.0/lib/rubocop/cop/cop.rb:128:in `add_offense'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:27:in `block in on_send'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:57:in `block in each_redundant_merge'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:19:in `redundant_merge_candidate'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:54:in `each_redundant_merge'
rubocop-0.60.0/lib/rubocop/cop/performance/redundant_merge.rb:26:in `on_send'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:58:in `block (2 levels) in trigger_responding_cops'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:106:in `with_cop_error_handling'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:57:in `block in trigger_responding_cops'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:56:in `each'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:56:in `trigger_responding_cops'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:34:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `block in on_begin'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `each'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `on_begin'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:100:in `on_def'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `block in on_begin'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `each'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:50:in `on_begin'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:150:in `on_while'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:150:in `on_while'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:150:in `on_while'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:35:in `block (2 levels) in <class:Commissioner>'
rubocop-0.60.0/lib/rubocop/ast/traversal.rb:13:in `walk'
rubocop-0.60.0/lib/rubocop/cop/commissioner.rb:46:in `investigate'
rubocop-0.60.0/lib/rubocop/cop/team.rb:116:in `investigate'
rubocop-0.60.0/lib/rubocop/cop/team.rb:104:in `offenses'
rubocop-0.60.0/lib/rubocop/cop/team.rb:44:in `inspect_file'
rubocop-0.60.0/lib/rubocop/runner.rb:269:in `inspect_file'
rubocop-0.60.0/lib/rubocop/runner.rb:216:in `block in do_inspection_loop'
rubocop-0.60.0/lib/rubocop/runner.rb:248:in `block in iterate_until_no_changes'
rubocop-0.60.0/lib/rubocop/runner.rb:241:in `loop'
rubocop-0.60.0/lib/rubocop/runner.rb:241:in `iterate_until_no_changes'
rubocop-0.60.0/lib/rubocop/runner.rb:212:in `do_inspection_loop'
rubocop-0.60.0/lib/rubocop/runner.rb:115:in `block in file_offenses'
rubocop-0.60.0/lib/rubocop/runner.rb:133:in `file_offense_cache'
rubocop-0.60.0/lib/rubocop/runner.rb:113:in `file_offenses'
rubocop-0.60.0/lib/rubocop/runner.rb:101:in `process_file'
rubocop-0.60.0/lib/rubocop/runner.rb:78:in `block in each_inspected_file'
rubocop-0.60.0/lib/rubocop/runner.rb:75:in `each'
rubocop-0.60.0/lib/rubocop/runner.rb:75:in `reduce'
rubocop-0.60.0/lib/rubocop/runner.rb:75:in `each_inspected_file'
rubocop-0.60.0/lib/rubocop/runner.rb:67:in `inspect_files'
rubocop-0.60.0/lib/rubocop/runner.rb:39:in `run'
rubocop-0.60.0/lib/rubocop/cli.rb:157:in `execute_runner'
rubocop-0.60.0/lib/rubocop/cli.rb:85:in `execute_runners'
rubocop-0.60.0/lib/rubocop/cli.rb:41:in `run'
rubocop-0.60.0/exe/rubocop:13:in `block in <top (required)>'
ruby-2.5.1/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'
rubocop-0.60.0/exe/rubocop:12:in `<top (required)>'
bin/rubocop:23:in `load'
bin/rubocop:23:in `<main>'
bin/ruby_executable_hooks:15:in `eval'
bin/ruby_executable_hooks:15:in `<main>'

Steps to reproduce the problem

def bar(**opts)
  opts.merge!(**foo)
  pp opts
end

RuboCop version

$> bundle exec rubocop -V
0.60.0 (using Parser 2.5.3.0, running on ruby 2.5.1 x86_64-darwin17)

Ruby version

$> ruby -v
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]