impl(pubsub): skip sending empty RPCs

[dbolduc]

Part of the work for #3957

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (97470cb) to head (efc5190).

Files with missing lines	Patch %	Lines
src/pubsub/src/subscriber/leaser.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4123      +/-   ##
==========================================
- Coverage   94.81%   94.76%   -0.06%     
==========================================
  Files         182      182              
  Lines        6887     6893       +6     
==========================================
+ Hits         6530     6532       +2     
- Misses        357      361       +4     

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