GHA: only try to publish docker image if secrets are set

[dweindl]

Another attempt to avoid failures on forks due to unavailable secrets. The previous check did not cover push-triggered runs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.77%. Comparing base (8b324bc) to head (cee7e43).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2310      +/-   ##
===========================================
- Coverage    77.78%   77.77%   -0.02%     
===========================================
  Files          318      318              
  Lines        20534    20534              
  Branches      1436     1436              
===========================================
- Hits         15973    15970       -3     
- Misses        4558     4561       +3     
  Partials         3        3              

Flag	Coverage Δ
cpp	73.56% <ø> (-0.02%)	⬇️
cpp_python	34.00% <ø> (ø)
petab	36.39% <ø> (ø)
python	72.33% <ø> (-0.01%)	⬇️
sbmlsuite	?

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see 3 files with indirect coverage changes