ci: Add explicit != '' comparisons to test if strings are non-empty

[jbencin]

Description

Fixes the docker push step of CI by explicitly checking if a string is empty using !=, rather than relying on an implicit cast

Additional info (benefits, drawbacks, caveats)

This fixes an issue introduced by #258. If you check here, when I made these changes they worked fine. I'm not sure what changed since them to make it fail now.

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[obycode]

LGTM. Thanks @jbencin!

[codecov]

Codecov Report

Merging #262 (1729198) into master (3a0ef04) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   93.49%   93.49%           
=======================================
  Files           6        6           
  Lines         338      338           
=======================================
  Hits          316      316           
  Misses         22       22           

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