[Bug] Allow handling of null for Convert of BaseConverterOneWay<TFrom, TTo>

[bijington]

In order to match a bug fixing coming to the MAUI Toolkit we should aim to solve it here also. The issue in question is:
CommunityToolkit/Maui#184

Description

Currently it is not possible to pass in a null value to the BaseConverterOneWay<TFrom, TTo> converter or any implementations of it due to checks within the base class.

Most important details from the discussion/issue:

• The TargetNullValue of XAML's Converter for bindings is applied to the result of the converter and not to the value before it goes in to the converter.
• Let the BaseConverterOneWay<TFrom, TTo> check for null and fallback to the default value for TFrom instead of throwing an Exception. (Suggested by @pictos )

• If we went with a blanket always replace nulls with default then that is a behavioral change but is still probably unlikely to have a big impact. If we added a way to opt-in as @Countryen suggested then it would have even less of an impact.

• This would then mean that the actual custom converter's ConvertFrom(TFrom value) is called with default(TFrom) when value is null. We could then add a null check to return something the view can use.

[Countryen]

I am currently trying to fix this. Please see the conversation in the MAUI issue, too.