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.

[Arlodotexe]

Source change looks good, test coverage looks good. Cloned and ran the tests locally in VS, all pass. Approved!