brp-fix-pyc-reproducibility: use more strict shell style
    
    1. Error out on unset variables.
    2. Implement suggestion made by shellcheck: use "||" instead of "-o"" in test.
    Shellcheck also suggested -print0 to avoid ambiguity with filenames with
    embedded newlines, but instead use '-exec {} +', which doesn't need xargs but
    still passes multiple arguments to a single marshalparser invocation.
    
    (Those issues are unlikely to cause problems in the rpm environment, but it's
    nice to be shellcheck-clean to use shellcheck during development.)
    
    Co-Authored-By: Benjamin A. Beasley <code@musicinmybrain.net>