Create parser error for empty TagHelper bound attribute values.

[NTaylorMullen]

Right now if a user creates a TagHelper and then provides it with an empty attribute value (ex: <myth myattr="" />) we rely on a C# compilation error to notify the user of their mistake. We could create a custom RazorError with more information associated with what went wrong.

Also note that this would only apply to TagHelpers attributes who don't happen to be typed as System.String.

[NTaylorMullen]

Wrong issue, reopening (mistake).