Fallback to empty array if branches is missing

[exoego]

When I enabled branch coverage in exoego/rspec-openapi#125, I found this line raises error because of coverage_branch_data is nil.

Sorry, I was not able to make a minimum repro nor make a test case, but this change is working in rspec-openapi repo.
Just an error log:

       Formatter SimpleCov::Formatter::CoberturaFormatter failed with NoMethodError: undefined method 'flat_map' for nil (/usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:275:in 'SimpleCov::SourceFile#build_branches')
       /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:275:in 'SimpleCov::SourceFile#build_branches': undefined method 'flat_map' for nil (NoMethodError)

             branches = coverage_branch_data.flat_map do |condition, coverage_branches|
                                            ^^^^^^^^^
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:99:in 'SimpleCov::SourceFile#branches'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:130:in 'SimpleCov::SourceFile#covered_branches'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:349:in 'SimpleCov::SourceFile#branch_coverage_statistics'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/source_file.rb:36:in 'SimpleCov::SourceFile#coverage_statistics'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:109:in 'block in SimpleCov::FileList#compute_coverage_statistics_by_file'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:108:in 'Array#each'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:108:in 'Enumerable#each_with_object'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:108:in 'SimpleCov::FileList#compute_coverage_statistics_by_file'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:31:in 'SimpleCov::FileList#coverage_statistics_by_file'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:115:in 'SimpleCov::FileList#compute_coverage_statistics'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/file_list.rb:27:in 'SimpleCov::FileList#coverage_statistics'
       	from /usr/local/lib/ruby/3.4.0/forwardable.rb:240:in 'SimpleCov::Result#coverage_statistics'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:43:in 'block in SimpleCov::ExitCodes::MinimumOverallCoverageCheck#calculate_minimum_violations'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:39:in 'Hash#each'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:39:in 'Enumerable#map'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:39:in 'SimpleCov::ExitCodes::MinimumOverallCoverageCheck#calculate_minimum_violations'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:35:in 'SimpleCov::ExitCodes::MinimumOverallCoverageCheck#minimum_violations'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/minimum_overall_coverage_check.rb:12:in 'SimpleCov::ExitCodes::MinimumOverallCoverageCheck#failing?'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/exit_code_handling.rb:11:in 'Array#each'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/exit_code_handling.rb:11:in 'Enumerable#find'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/exit_codes/exit_code_handling.rb:11:in 'SimpleCov::ExitCodes::ExitCodeHandling.call'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov.rb:262:in 'SimpleCov.result_exit_status'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov.rb:249:in 'SimpleCov.process_result'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov.rb:234:in 'SimpleCov.process_results_and_report_error'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov.rb:192:in 'SimpleCov.run_exit_tasks!'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov.rb:179:in 'SimpleCov.at_exit_behavior'
       	from /usr/local/bundle/gems/simplecov-0.22.0/lib/simplecov/defaults.rb:30:in 'block in <top (required)>'
     # ./spec/spec_helper.rb:13:in 'SpecHelper#assert_run'
     # ./spec/spec_helper.rb:23:in 'block (2 levels) in SpecHelper#run_tests'
     # ./spec/spec_helper.rb:22:in 'Dir.chdir'
     # ./spec/spec_helper.rb:22:in 'block in SpecHelper#run_tests'
     # ./spec/spec_helper.rb:21:in 'Kernel#public_send'
     # ./spec/spec_helper.rb:21:in 'SpecHelper#run_tests'
     # ./spec/spec_helper.rb:29:in 'SpecHelper#rspec'
     # ./spec/rspec/rack_test_spec.rb:30:in 'block (3 levels) in <top (required)>'