Ignore X.509 users in scram secret collision validation

[verdel]

Summary

During validation, users without the scramCredentialsSecretName parameter are added to a dictionary with an empty key, which leads to the error mentioned in issue #122.

Closes #122

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[nammn]

evergreen retry

[MaciejKaras]

LGTM!

[verdel]

@MaciejKaras, Is there anything I can help with to get this PR reviewed faster?

[lsierant]

LGTM!