Shebangs check reports false positives for packages requiring /usr/bin/env

[hroncok]

See https://taskotron-dev.fedoraproject.org/artifacts/all/df5874c4-7b70-11e7-9854-52540009effe/task_output/output.log

These RPMs contain problematic shebang in some of the scripts:
pwhois-2.99.010-1.fc27
 * Scripts containing `#!/usr/bin/env python` shebang:
   
This is discouraged and should be avoided. Please check the shebangs
and use either `#!/usr/bin/python2` or `#!/usr/bin/python3`.

The package requires /usr/bin/env, so I guess that's the problem.

[hroncok]

This adds an empty set in such case:

taskotron-python-versions/taskotron_python_versions/unversioned_shebangs.py

Line 55 in c2b8df0

scripts_summary[shebang] = get_problematic_files(

    for shebang in FORBIDDEN_SHEBANGS:
        if shebang_to_require(shebang) in package.require_names:
            scripts_summary[shebang] = get_problematic_files(
                package.path, shebang)

I suggest we change the code to only set scripts_summary[shebang] if get_problematic_files(...) returned some problematic files.