ignore nil branches during parallel combine

[shixianyangidme]

This issue had very similar cause of another pending PR: , but it is caused by parallel test. My project is based on ruby 3.1.4
If a parallel test (2 processes) generates two results on the same file:
Result A
"/Users/shixian.yang/workspace/test.rb": { "lines": [ 1, 1, null ] },
Result B
"/Users/shixian.yang/workspace/test.rb": { "lines": [ 0, 0, null ] },
Neither of results have "branches", the merge will generate a "branches": nil as a result. Finally it ends up in the error
NoMethodError: undefined method `flat_map' for nil:NilClass.

My plan is to ignore the "branches" field if neither A and B have "branches" exist.