fix: Handle no current owner in GraphQL resolvers

[scott-codecov]

Purpose/Motivation

This could happen if you're logged in as a user who has 1 linked owner on service A and then you try and load a public repo on service B.

Links to relevant tickets

Saw 2 instances of this in Sentry so far: https://codecov.sentry.io/issues/4328543223/

What does this PR do?

Handles no current_owner in the case described above.

[codecov]

Codecov Report

Merging #21 (f3abc18) into main (f081451) will not change coverage.
The diff coverage is 100.00%.

@@          Coverage Diff          @@
##            main     #21   +/-   ##
=====================================
  Coverage   95.23   95.23           
=====================================
  Files        559     559           
  Lines      14025   14030    +5     
=====================================
+ Hits       13356   13361    +5     
  Misses       669     669           

Flag	Coverage Δ
unit	95.23% <100.00%> (+<0.01%)	⬆️
unit-latest-uploader	95.23% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../owner/interactors/get_is_current_user_an_admin.py	96.96% <100.00%> (+0.19%)	⬆️
graphql_api/actions/repository.py	100.00% <100.00%> (ø)
graphql_api/types/owner/owner.py	91.52% <100.00%> (+0.14%)	⬆️