agreement: add omitempty to MessageHandle in agreement message

[iansuvak]

Summary

As listed in comments and in the issue below we want to remove no longer necessary field from the struct. We will still need another PR and another consensus release before it's safe to delete but we need to add omitempty first directive to the field only which this PR does. Not including omitempty was an overlook on the original PR was an overlook and I didn't know that the directive could be applied to a single field. Only noticed it recently while working on other unmarshalling msgp code.

Once the consensus release is out we can actually delete the field and the associated tests as it will be guaranteed that no nodes running agreement will have this field encoded in their crashDB.

Test Plan

In addition to modifying the existing test below, also ran across 364 rows obtained from a testnet node instrumented to not overwrite the data row and instead append to it. It successfully decoded all of the rows and re-encoded them without including references to MessageHandle. For more details about the test including a screenshot see the original PR at #4644

[codecov]

Codecov Report

Merging #5593 (25a4a97) into master (d316914) will increase coverage by 0.00%.
Report is 23 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5593   +/-   ##
=======================================
  Coverage   55.79%   55.79%           
=======================================
  Files         446      446           
  Lines       63418    63418           
=======================================
+ Hits        35381    35384    +3     
+ Misses      25668    25667    -1     
+ Partials     2369     2367    -2     

Files Changed	Coverage Δ
agreement/message.go	71.42% <ø> (ø)

... and 11 files with indirect coverage changes

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