- #4988
- preserve whitespace as the setting demands
- correct previous `string.IsNullOrEmpty()` call to match previous `ValueProviderResultExtensions.ConvertTo()` use
- short-circuit other `string`-to-`string` conversions (as `ValueProviderResultExtensions.ConvertTo()` does)
- correct documentation of `ConvertEmptyStringToNull` properties
- add more tests of these scenarios and remove duplicate `BindModel_ValidValueProviderResult_ConvertEmptyStringsToNull()` test
