Added check of RequiresValidationContext before calling IsValid

[vvg88]

The fix is done to avoid NRE when CompareAttribute's implementation of IsValid(object? value, ValidationContext validationContext) is called with validationContext == null.

Fix #42490

Found by Linux Verification Center (linuxtesting.org) with SVACE.

[tarekgh]

Thank you for submitting the PR. Here are a few points regarding this change:

• According to ValidationAttribute.RequiresValidationContext), it states: Gets a value that indicates whether the attribute requires validation context. Therefore, in my opinion, the correct behavior is to throw an exception when validating without providing the context. Allowing IsValid to return false when RequiresValidationContext is true masks the issue, failing to educate callers on the need to provide the context when using such a validation attribute. Additionally, it would not make it clear why the validation failed.
• Any change here must be considered a breaking change, as we cannot predict how various users might have implemented their custom attributes.
• Any proposed change should also include appropriate test cases.

CC @eiriktsarpalis @jeffhandley

[vvg88]

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[vvg88]

@tarekgh Your comments have been addressed, could you please take a look?

[vvg88]

@adamsitnik Hello! Could you please help to find a reviewer of this PR?
Thanks!
CC @jeffhandley

[tarekgh]

@vvg88 thanks for helping with this change. Sorry for the delay reviewing it. You know this is holiday time and most people are off.