VDiff: Make max diff duration upgrade/downgrade safe

[mattlord]

Description

In v19 we added a new feature to support auto restarting the diff of a table periodically in #14786.

The proto field used for that is a vttime.Duration type. Because it ends up being a pointer in the proto message there is no ZeroValue in the server side (vtctld) message when the client (vtctldclient) sent an older version of the VDiffCreateRequest proto message from a v18 vtctldclient binary and thus when we passed on req.MaxDiffDuration.Seconds to the tablets we got a nil pointer exception.

This PR adds a server side nil check for this new field in the server's message and sets it to a pointer to the type's ZeroValue so that this is upgrade/downgrade safe.

Related Issue(s)

• Fixes: Bug Report: vdiff create panics when using a v18 vtctldclient with v19 vtctld #14996

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (737fdb3) 47.31% compared to head (1a92ab9) 47.34%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14995      +/-   ##
==========================================
+ Coverage   47.31%   47.34%   +0.02%     
==========================================
  Files        1142     1142              
  Lines      239009   239018       +9     
==========================================
+ Hits       113091   113166      +75     
+ Misses     117327   117258      -69     
- Partials     8591     8594       +3     

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