Fix plugin crash when variable is passed for ConnectionPool

[niveathika]

Purpose

$subject

Part of ballerina-platform/ballerina-library#2536

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #316 (2f8357f) into master (ad26c68) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #316   +/-   ##
=========================================
  Coverage     86.02%   86.02%           
  Complexity       17       17           
=========================================
  Files             6        6           
  Lines            93       93           
  Branches         23       23           
=========================================
  Hits             80       80           
  Misses           10       10           
  Partials          3        3           

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