agreement: don't attempt to make proposal if no particating accounts

[cce]

Summary

As described in #590 if you don't have any participation keys, you don't need to make a proposal. This adds a check to skip calling AssembleBlock() if there are no participating accounts.

Test Plan

Existing tests should pass, also need to write new test asserting AssembleBlock is not called.

[codecov]

Codecov Report

Attention: Patch coverage is 39.13043% with 14 lines in your changes missing coverage. Please review.

Project coverage is 51.86%. Comparing base (64ae8b0) to head (75daeed).

Files with missing lines	Patch %	Lines
data/pools/transactionPool.go	0.00%	8 Missing ⚠️
agreement/pseudonode.go	0.00%	1 Missing and 1 partial ⚠️
components/mocks/mockParticipationRegistry.go	0.00%	2 Missing ⚠️
data/account/participationRegistry.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4976   +/-   ##
=======================================
  Coverage   51.86%   51.86%           
=======================================
  Files         653      653           
  Lines       87547    87559   +12     
=======================================
+ Hits        45408    45416    +8     
- Misses      39264    39272    +8     
+ Partials     2875     2871    -4     

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