disallow cred helper to return empty output

[dberenbaum]

Continuing from #220 (comment), the issue appears to be from having the cache helper enabled in my git config:

[credential]
	helper = cache

I get the prompt as expected when I disable this. However, when it's enabled, I don't get a prompt, even if I have no credentials cached. For example, this fails:

$ git credential-cache exit
$ dvc get -v https://github.com/dberenbaum/config-files vimrc

The credential helper does not fail but returns an empty output in this scenario.

I notice we don't have any tests for any of the credential helper logic. Do we need to add something?

I don't mind if you take this over @pmrowla, but once I found the issue it wasn't any extra work to submit the PR.

[codecov-commenter]

Codecov Report

Patch coverage: 94.73% and project coverage change: +1.64 🎉

Comparison is base (23b23dc) 79.60% compared to head (17c8a9b) 81.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   79.60%   81.25%   +1.64%     
==========================================
  Files          26       27       +1     
  Lines        3776     3814      +38     
  Branches      663      668       +5     
==========================================
+ Hits         3006     3099      +93     
+ Misses        675      608      -67     
- Partials       95      107      +12     

Impacted Files	Coverage Δ
src/scmrepo/git/credentials.py	52.19% <66.66%> (+26.06%)	⬆️
tests/test_credentials.py	100.00% <100.00%> (ø)

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