Fix compiler plugin failure when the diagnostic code is null

[niveathika]

Purpose

$subject

Related to ballerina-platform/ballerina-library#4054

Checklist

• Linked to an issue
• Updated the specification
• Updated the changelog
• Added tests
• Checked native-image compatibility

[codecov]

Codecov Report

Base: 82.49% // Head: 82.49% // No change to project coverage 👍

Coverage data is based on head (3ea9ee8) compared to base (a786041).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##             2201.3.x     #616   +/-   ##
===========================================
  Coverage       82.49%   82.49%           
  Complexity        780      780           
===========================================
  Files              20       20           
  Lines            3171     3171           
  Branches          485      485           
===========================================
  Hits             2616     2616           
  Misses            391      391           
  Partials          164      164           

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