fix: allow publish to topic without sender

[mattjohnsonpint]

Publishing a message from an actor to a topic works fine:

err := senderActorPid.Tell(ctx, topicActorPid, publishMessage)

But publishing anonymously was creating a panic that would shut down the actor system:

import goakt "github.com/tochemey/goakt/v3/actor"

...

err := goakt.Tell(ctx, topicActorPid, publishMessage)

The panic was due to ctx.Sender() being nil in the handlePublish method.

Fixed. Copied and modified one of the existing test, which would fail without the fix but now passes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.02%. Comparing base (e148d09) to head (2699566).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #761      +/-   ##
==========================================
- Coverage   88.12%   88.02%   -0.10%     
==========================================
  Files         101      101              
  Lines        8780     8783       +3     
==========================================
- Hits         7737     7731       -6     
- Misses        819      826       +7     
- Partials      224      226       +2     

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[Tochemey]

Good catch