Add Color-to-String Unit Tests

[TheCodeTraveler]

Description of Change

This PR implements the missing tests for the following ColorTo[Type]StringConverters:

• ColorToRgbStringConverter
• ColorToRgbaStringConverter
• ColorToHexRgbStringConverter
• ColorToHexRgbaStringConverter
• ColorToCmykStringConverter
• ColorToCmykaStringConverter

Linked Issues

• Fixes [Proposal] Color-To-string Converters #54

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

Additional information

BaseConverter<TFrom, TTo>

• Added ArgumentNullException.ThrowIfNull(targetType); to the following methods:
  • BaseConverter<TFrom, TTo>.Convert()
  • BaseConverter<TFrom, TTo>.ConvertBack()
• The targetType of an IValueConverter should never be null because a value requires a non-null Type to be assigned to
• Updated Unit Tests accordingly

IsEqualConverter_Tests

• Removed IsNotEqualConverter unit tests from IsEqualConverter_Tests.cs
  • The IsNotEqualConverter unit tests are already covered in IsNotEqualConverter_Tests.cs

StringToListConverter_Tests

• Fixed tests that were incorrectly testing ListToStringConverter
  • Was likely a copy/paste error from ListToStringConverter_Tests

IsEqualConverter + IsNotEqualConverter

• Use EqualityComparer to compare values.

• EqualityComparer is the recommended approach for comparing values and is more thorough than the previous implementation that only used .Equals():

  The Default property checks whether type T implements the System.IEquatable interface and, if so, returns an EqualityComparer that uses that implementation. Otherwise, it returns an EqualityComparer that uses the overrides of Object.Equals and Object.GetHashCode provided by T.

  https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.equalitycomparer-1.default?view=net-6.0#remarks

StringToListConverter

• Change TTo from IEnumerable -> IEnumerable<string>
  • The existing logic for StringToListConverter always returns IEnumerable<string>.