Update ValueConverterExtension to allow for comparison of Nullable types to corresponding underlying types

[BruceTheBrick]

This PR aims to resolve the issues outlined in #1300
and align the behavior to comparable converters from Xamarin.Forms.

[BruceTheBrick]

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[VladislavAntonyuk]

Could you please add Sample? The other changes look good.

[BruceTheBrick]

Could you please add Sample? The other changes look good.

Good call man, thank you!

Just some questions before this PR gets merged:

1. Given that I modified the ValueConverterExtension.shared.cs which I believe is used by many of the other converters, would it be best for me to also update all their test classes with additional tests of a similar nature to those already added in IsStringNotNullOrEmptyConverterTests.cs?
2. Would it be best for me to add similar samples to the other converter sample pages also?

Thanks man!

[VladislavAntonyuk]

1. As you have covered that part of code, I don't think it needs to be duplicated for all converters since they use the same code.
2. The same for samples.

@brminnick what do you think?

[brminnick]

I always say that the more tests the better! Bruce, if you're able to add similar unit tests for the other converters, that'd be incredible 💯

[BruceTheBrick]

Thanks @brminnick & @VladislavAntonyuk!
Additional tests added to other relevant converters.

[brminnick]

Awesome!! Thanks @BruceTheBrick!

@VladislavAntonyuk - I won't be able to review this until next week, but if you happen to have the time for one more review, we could merge this and fold it into the next week's v5.3.0 release!

And no worries if not! We can release it in v6.0.0 next month.

[brminnick]

Thanks @BruceTheBrick!!

Apologies - It turns out I was wrong in my previous review.

I tightened up the logic in ValueConverterExtension.ValidateTargetType() to ensure that we're only enabling Nullable Value Types for TTo and not TFrom (mostly because I'm unsure if that would be a breaking change).

I added your new sample to IsStringNullOrEmptyConverterPage as well.

[BruceTheBrick]

Thanks @BruceTheBrick!!

Apologies - It turns out I was wrong in my previous review.

I tightened up the logic in ValueConverterExtension.ValidateTargetType() to ensure that we're only enabling Nullable Value Types for TTo and not TFrom (mostly because I'm unsure if that would be a breaking change).

I added your new sample to IsStringNullOrEmptyConverterPage as well.

Thanks for all your work there @brminnick, I really appreciate it!
I appreciate the support that yourself and @VladislavAntonyuk provided in getting this sorted out.