Fix line number for empty file offense

An offense added by a cop to an empty file with have line number = 1.
For example:

```ruby
RSpec.describe RuboCop::Cop::Style::Copyright, :config do
  context 'when the source code file is empty' do
    it 'registers an offense' do
      expect_offense('^ Include a copyright notice matching [...]')
    end
  end
end
```

Ensure the line number matches for `expect_offense` on an empty file.

lib/rubocop/rspec/expect_offense.rb

@@ -209,6 +209,7 @@ def self.parse(annotated_source)
               source << source_line
             end
           end
+          annotations.each { |a| a[0] = 1 } if source.empty?
 
           new(source, annotations)
         end