Fix Performance/RedundantStringChars cop error in case of implicit …

…receiver

```console
echo 'chars.size' | be rubocop  --only Performance/RedundantStringChars --stdin bug.rb -d

Inspecting 1 file
Scanning /bug.rb
An error occurred while Performance/RedundantStringChars cop was inspecting /bug.rb:1:0.
undefined method `begin_pos' for nil
/lib/rubocop/cop/performance/redundant_string_chars.rb:76:in `correction_range'
/lib/rubocop/cop/performance/redundant_string_chars.rb:62:in `block in on_send'
```

changelog/fix_performance_redundant_string_chars_cop_error_in_case_of_implicit_receiver.md

@@ -0,0 +1 @@
+* [#478](https://github.com/rubocop/rubocop-performance/pull/478): Fix `Performance/RedundantStringChars` cop error in case of implicit receiver. ([@viralpraxis][])

lib/rubocop/cop/performance/redundant_string_chars.rb

@@ -48,7 +48,7 @@ class RedundantStringChars < Base
         RESTRICT_ON_SEND = %i[[] slice first last take length size empty?].freeze
 
         def_node_matcher :redundant_chars_call?, <<~PATTERN
-          (send $(send _ :chars) $_ $...)
+          (send $(send !nil? :chars) $_ $...)
         PATTERN
 
         def on_send(node)

spec/rubocop/cop/performance/redundant_string_chars_spec.rb

@@ -139,4 +139,10 @@
       str.chars.max
     RUBY
   end
+
+  it 'does not register an offense with implicit receiver' do
+    expect_no_offenses(<<~RUBY)
+      chars.size
+    RUBY
+  end
 end