Fix Not Allowing to Pass null as Variable Input Value

[DimuthuMadushan]

Purpose

Fixes: #2696

Examples

N/A

Checklist

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

[codecov]

Codecov Report

Merging #616 (8de4b58) into master (834fee6) will increase coverage by 0.01%.
The diff coverage is 97.56%.

@@             Coverage Diff              @@
##             master     #616      +/-   ##
============================================
+ Coverage     93.89%   93.91%   +0.01%     
  Complexity      347      347              
============================================
  Files            54       54              
  Lines          3671     3682      +11     
  Branches       1998     2006       +8     
============================================
+ Hits           3447     3458      +11     
  Misses          197      197              
  Partials         27       27              

Impacted Files	Coverage Δ
ballerina/validator_visitor.bal	96.35% <95.00%> (+0.07%)	⬆️
ballerina/common_utils.bal	94.44% <100.00%> (+0.02%)	⬆️
ballerina/variable_validator.bal	98.25% <100.00%> (+<0.01%)	⬆️

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