Fix warn tx pool endpoint

[miiu96]

Reasoning behind the pull request

• Added an extra check for sender address, for the case when we have rewards transactions in pool and the /transaction/pool endpoint is called. ( the field sender for a reward transaction is empty)

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.51%. Comparing base (d8d6658) to head (8ef17e0).
⚠️ Report is 5 commits behind head on feat/supernova-async-exec.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feat/supernova-async-exec    #7691      +/-   ##
=============================================================
- Coverage                      77.51%   77.51%   -0.01%     
=============================================================
  Files                            880      880              
  Lines                         122572   122576       +4     
=============================================================
- Hits                           95017    95014       -3     
- Misses                         21229    21234       +5     
- Partials                        6326     6328       +2     

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