Description of problem:
When %py3_shebang_flags is %nil, %py3_check_import evals to:

...
/usr/bin/python3 - /usr/lib/rpm/redhat/import_all_modules.py
...

And hence it executes no code.

When %py3_shebang_flags is undefined, %py3_check_import evals to:

...
/usr/bin/python3 -%{py3_shebang_flags} /usr/lib/rpm/redhat/import_all_modules.py
...

And hence fails.

The -%{py3_shebang_flags} flag should be conditionalized on %{?py3_shebang_flags} non-emptines. %py3_shebang_fix already does something like that.

Version-Release number of selected component (if applicable): 
python-rpm-macros-3.9-19.fc33
python-rpm-macros-3.9-40.fc34
python-rpm-macros-3.10-9.fc35
python-rpm-macros-3.10-12.fc36