gh-136264: Fix PEP-739 build-details generation with relative-paths

[itamaro]

there were two issues with the make_paths_relative implementation:

1. for top-level entries (i.e. base_interpreter), the parent is "", leading to the part loop to set the container to the non-existent top-level "" key, which becomes {} because it's a defaultdict
2. for dotted-entries that don't exist in the data, current_path ends up being {}, again, because it's a defaultdict

this PR proposes one (granted, somewhat sus) way to fix it, by:

1. checking the truthyness of part before indexing into the container (fixing the first issue)
2. explicitly raising the KeyError when it would have been otherwise raised by a non-defaultdict dict (fixing the second issue)

• Issue: PEP-739 build-details generation is broken when --relative-paths is specified #136264

[AA-Turner]

Closed by #138510.

A