Online DDL: do not update last_throttled_timestamp with zero value

[shlomi-noach]

Description

_vt.schema_migrations.last_throttled_timestamp indicates the last time a vreplication migration was throttled, value copied from _vt.vreplication.time_throttled. However, it is possible for the value to be zero if timeThrottled is uninitialized. In this PR we avoid updating last_throttled_timestamp to the zero value.

Related Issue(s)

Fixes #16394

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[rohit-nayak-ps]

It might be a good idea to initialize one of the OnlineDDL tests with the strict modes that Vitess might run with.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.67%. Comparing base (3cb61cd) to head (b8a9654).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16395      +/-   ##
==========================================
- Coverage   68.68%   68.67%   -0.02%     
==========================================
  Files        1548     1548              
  Lines      199084   199085       +1     
==========================================
- Hits       136736   136712      -24     
- Misses      62348    62373      +25     

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[shlomi-noach]

It might be a good idea to initialize one of the OnlineDDL tests

Good idea, done.

[shlomi-noach]

(they ought to have such sql_mode by default, since this is 8.0, but now explicitly setting it like so)