Rename IsNullOrEmptyConverter to IsStringNullOrEmptyConverter

[brminnick]

Description of Change

This PR renames the following converters:

1. Rename IsNullOrEmptyConverter to IsStringNullOrEmptyConverter
   • Throw an InvalidArgumentException if object? value is not string
   • Use string.IsNullOrEmpty to evaluate value
2. Rename IsNotNullOrEmptyConverter to IsStringNotNullOrEmptyConverter
   • Throw an InvalidArgumentException if object? value is not string
   • Use string.IsNullOrEmpty to evaluate value

Linked Issues

• Fixes isNullOrEmptyConverter should check for empty instead of WhiteSpace #278 (comment)

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)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls/XYZ

[bijington]

Awesome work @brminnick

[IeuanWalker]

@brminnick Hi, think this change has broken my use case.

Was using this converter to check if double? was null or not, but now I'm getting a InvalidCastException

Any chance of a converter to check if an object? is null or not. I'm happy to do a PR for it.

[bijington]

@IeuanWalker I suspect we will need an IsNullConverter and IsNotNullConverter given this change as you have pointed out. I don't think it would need to be IsObjectNullConverter but I am open to opinions here.

I don't know what the correct approach is here in terms of opening a proposal to cover this work or not. I shall delegate to the rest of the team on this one: @jfversluis @VladislavAntonyuk @pictos @brminnick

[VladislavAntonyuk]

@IeuanWalker I suggest to create a discussion. The new control by its name can be used for String only.
As an alternative you can use EqualConverter and check if value is null.
But in discussion we can discuss if we need a new IsNullConverter or we replace casting with System.Convert or we leave it as it is.

[IeuanWalker]

@VladislavAntonyuk will open a discussion now.

I tried using the NotEqualConverter but doesn't seem to work for null checking