Suppress Style/EmptyHeredoc offenses

This commit suppresses the following `Style/EmptyHeredoc` offenses.

```cnosole
% bundle exec RuboCop
(snip)

Offenses:

spec/rubocop/ast/processed_source_spec.rb:385:22: C: [Correctable]
Style/EmptyHeredoc: Use an empty string literal instead of heredoc.
      let(:source) { <<~RUBY }
                     ^^^^^^^
spec/rubocop/ast/processed_source_spec.rb:406:22: C: [Correctable]
Style/EmptyHeredoc: Use an empty string literal instead of heredoc.
      let(:source) { <<~RUBY }
                     ^^^^^^^

162 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

spec/rubocop/ast/processed_source_spec.rb

@@ -382,8 +382,7 @@ def some_method
 
   describe '#blank?' do
     context 'with source of no content' do
-      let(:source) { <<~RUBY }
-      RUBY
+      let(:source) { '' }
 
       it 'returns true' do
         expect(processed_source).to be_blank
@@ -403,8 +402,7 @@ def some_method
 
   describe '#start_with?' do
     context 'with blank source' do
-      let(:source) { <<~RUBY }
-      RUBY
+      let(:source) { '' }
 
       it 'returns false' do
         expect(processed_source).not_to be_start_with('start')