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: 90.60% // Head: 90.60% // No change to project coverage 👍

Coverage data is based on head (5802a58) compared to base (99a1d4b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##             2201.3.x     #814   +/-   ##
===========================================
  Coverage       90.60%   90.60%           
  Complexity         37       37           
===========================================
  Files              13       13           
  Lines             266      266           
  Branches           44       44           
===========================================
  Hits              241      241           
  Misses             10       10           
  Partials           15       15           

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