[release-20.0] Fix panic in schema tracker in presence of keyspace routing rules (#16383)

[GuptaManan100]

Description

This is a backport of #16383

Related Issue(s)

• Fixes Bug Report: Panic in vtgate schema manager because of keyspace rules #16382

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.66%. Comparing base (5289c44) to head (e8e9624).

Files	Patch %	Lines
go/vt/vtgate/vschema_manager.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16405      +/-   ##
================================================
+ Coverage         68.65%   68.66%   +0.01%     
================================================
  Files              1541     1541              
  Lines            197848   197848              
================================================
+ Hits             135835   135857      +22     
+ Misses            62013    61991      -22     

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