Fix Style/CaseEquality when AllowOnConstant is true

When the method receiver is implicit it was failing with a NoMethodError
in `nil` error.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#7882](https://github.com/rubocop-hq/rubocop/pull/7882): Fix `Style/CaseEquality` when `AllowOnConstant` is `true` and the method receiver is implicit. ([@rafaelfranca][])
+
 ## 0.82.0 (2020-04-16)
 
 ### New features

lib/rubocop/cop/style/case_equality.rb

@@ -42,7 +42,7 @@ def on_send(node)
 
         def const?(node)
           if cop_config.fetch('AllowOnConstant', false)
-            !node.const_type?
+            !node&.const_type?
           else
             true
           end

spec/rubocop/cop/style/case_equality_spec.rb

@@ -21,6 +21,12 @@
       )
     end
 
+    it 'does not fail when the receiver is implicit' do
+      expect_no_offenses(<<~RUBY)
+        puts "No offense"
+      RUBY
+    end
+
     it 'does not register an offense for === when the receiver is a constant' do
       expect_no_offenses(<<~RUBY)
         Array === var