Fix Target Type Validation

[TheCodeTraveler]

Description of Change

This fix updates the logic for IsValidTargetType(Type, bool) to use the targetType parameter passed into the method when verifying the nullable value types.

Linked Issues

• Fixes [BUG] Non-nullable are not valid InvalidOperationException is thrown when using VariableMultiValueConverter #2003

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

I think this is the fix. But I'll defer to @bijington as the expert here since he added the nullable value type check in this PR: #1926

The exception being thrown in #2003 was caused by using MultiBindings where TTo may not match Type targetType.

[bijington]

Looks good to me @brminnick thanks for finishing this up. I just added the sample issue into the sample application as that was how I was testing this. I see you have a unit test covering it also though

[ennethtpz]

Hey, when will the new version of MAUI toolkit come out? We need this fix as soon as we can.