Clarify docs around `ValidatingModelBase.GetErrors(null)`

Hi,
Method ValidatingModelBase.GetErrors(string propertyName) supposed to return entity-level errors if propertyName is null or string.empty (as it said in description):
`The name of the property to retrieve validation errors for; or null or System.String.Empty, to retrieve entity-level errors.`

However, it's always returns **null** if no propertyName is set.

Locate ValidatingModelBase.GetErrors(string propertyName) and read its implementation alongside the XML documentation quoted in the issue. Verify the behavior for null and string.Empty, then align the documentation or behavior with the intended entity-level error contract; done when the documented and observed behavior agree.

Written by the indexing model from the issue text.