Always return a valid timezone in cursor

[dbussink]

Conversion functions into a Go time.Time expect to have a valid *time.Location and will panic if passed in nil.

Before we used also here nil to signal no timezone is set (and thus falling back to time.Local implicitly, but it's better to always return a timezone explicitly and use time.Local where appropriate then.

Related Issue(s)

Fixes #17545

Marked also for back porting as this can panic.

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• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.70%. Comparing base (301c92c) to head (f33200f).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/executorcontext/safe_session.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17546      +/-   ##
==========================================
+ Coverage   67.68%   67.70%   +0.01%     
==========================================
  Files        1584     1584              
  Lines      254717   254718       +1     
==========================================
+ Hits       172417   172465      +48     
+ Misses      82300    82253      -47     

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