Fix optimized DP setter codegen for 'string' properties

[Sergio0694]

Follow-up to #792.

The optimized codegen for generated dependency property setters introduced in #792 calls XamlBindingHelper.SetProperty* directly, but it turns out that XamlBindingHelper.SetPropertyFromString does not work correctly when the value is null or an empty string. To work around this, the generator now special-cases string properties and emits this code instead of an unconditional helper call:

if (value is null || value.Length == 0)
{
    SetValue(TestProperty, value);
}
else
{
    XamlBindingHelper.SetPropertyFromString(this, TestProperty, value);
}

This is applied to both the local-caching and no-caching setter paths.

The existing unit tests covering string and string? dependency properties (including the parameterized rows in SingleProperty_MultipleTypes_WithNoCaching_DefaultValueIsOptimized and SingleProperty_WithCustomMetadataType_WithNoCaching, plus the explicit SingleProperty_String_WithLocalCache test and others) have been updated to reflect the new generated code, so coverage for both setter paths is preserved.

[michael-hawker]

Suggested change

	if (value is null || value.Length == 0)
	if (string.IsNullOrEmpty(value))

QQ: I know it's equivalent but would using the string helper be clearer/more efficient in anyway? (i.e. would it be optimized by the compiler anyway vs. the function overhead?)