Lint/EmptyFile throws error when html formatter is used

[roberts1000]

Lint/EmptyFile (added in 0,90.0) throws an error when the html formatter is used.

Steps to Reproduce

1. Enable the cop in a .rubocop.yml

   Lint/EmptyFile:
     Enabled: true

2. Ensure the project has an empty .rb file.

3. Run

   $ rubocop --format html --out results.html

   The following error occurs:

   index 0 outside of array bounds: 0...0
   /home/dev/.rvm/gems/ruby-2.6.6/gems/parser-2.7.1.4/lib/parser/source/buffer.rb:270:in `fetch'
   ...
   

4. Run

   $ rubocop

   It works as expected.

Environment

$ [bundle exec] rubocop -V
0.90.0 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.6.6 x86_64-linux)