Fix 'generated_antlr?' to handle cases where '.g' files container fewer than 2 lines

[lildude]

Description

As reported in #5264, analysing a repo with a file with a .g extension that contains no lines will fail with:

`ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/generated.rb:451:in `generated_antlr?': undefined method `include?' for nil:NilClass (NoMethodError)
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/generated.rb:86:in `generated?'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/generated.rb:12:in `generated?'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/blob_helper.rb:359:in `generated?'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/lazy_blob.rb:52:in `generated?'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/blob_helper.rb:382:in `include_in_language_stats?'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/repository.rb:164:in `block in compute_stats'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/repository.rb:149:in `each_delta'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/repository.rb:149:in `compute_stats'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/repository.rb:116:in `cache'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/lib/linguist/repository.rb:98:in `breakdown_by_file'
	from /usr/local/rvm/rubies/ruby-2.6.3/lib/ruby/gems/2.6.0/gems/github-linguist-7.13.0/bin/github-linguist:48:in `<top (required)>'
	from /usr/local/rvm/rubies/ruby-2.6.3/bin/github-linguist:23:in `load'
	from /usr/local/rvm/rubies/ruby-2.6.3/bin/github-linguist:23:in `<main>'
	from /usr/local/rvm/rubies/ruby-2.6.3/bin//ruby_executable_hooks:24:in `eval'
	from /usr/local/rvm/rubies/ruby-2.6.3/bin//ruby_executable_hooks:24:in `<main>'`

This is because generated_antlr? doesn't cater for scenarios where these files may be empty or container few than 2 lines - it tests for a specific string on the second line.

This PR fixes that.

[Rest of template removed as it doesn't apply]

/cc @Nixinova as the author of #5199 which added this method.