Fix for custom convertor returning null throws NRE. #220

[moxplod]

Description

If the custom convertor returned null. That threw an NRE.

Fixes # #220

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

[what-the-diff]

PR Summary

• Enhancement in BaseQueryBuilder.cs
  Adjustments were made in the query builder to better handle instances when the AllowNullSearch configuration is set to null. This improvement ensures that the search function can accommodate such scenarios without causing any unexpected errors or behaviors.

• Addition of a Test Scenario in GridifyExtensionsShould.cs
  A new test has been added to validate custom conversion circumstances. This test is designed to verify if it correctly returns a null value when the input specifically is "null". This addition is crucial to making sure our conversion process handles all edge-cases and works reliably under different conditions.

[alirezanet]

LGTM 👌