Fix incorrect return statement in auth (#2086)

[kamyabzad]

Pull Request check-list

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Was the change added to CHANGES file?

Description of change

Fix a bug in commands.core.ManagementCommands.auth that returned a function handler when provided with a None username

[codecov-commenter]

Codecov Report

Merging #2092 (226eb22) into master (cb91eed) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2092   +/-   ##
=======================================
  Coverage   92.45%   92.46%           
=======================================
  Files         104      104           
  Lines       24364    24365    +1     
=======================================
+ Hits        22526    22528    +2     
+ Misses       1838     1837    -1     

Impacted Files	Coverage Δ
redis/commands/core.py	84.04% <100.00%> (+0.03%)	⬆️
tests/test_commands.py	94.32% <100.00%> (+<0.01%)	⬆️

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[chayim]

@kamyabzad thanks for the contribution! Curious what your thoughts are on the suggestion above

[kamyabzad]

@chayim Thanks for the suggestion, I like it more. I have updated the code and the tests (also rebased onto upstream).

[chayim]

@kamyabzad This looks great! I think it's good to merge into the next release.

Thanks again, for both pointing out the issue, and fixing it!

[kamyabzad]

@kamyabzad This looks great! I think it's good to merge into the next release.

Thanks again, for both pointing out the issue, and fixing it!