true if the value parameter is null or string.Empty, or if value consists exclusively of white-space characters.
string.IsNullOrWhiteSpace(string) is a convenience method that is similar to the following code, except that it offers superior performance:
code reference: System.String.IsNullOrWhitespace#2
White-space characters are defined by the Unicode standard. The string.IsNullOrWhiteSpace(string) method interprets any character that returns a value of true when it is passed to the char.IsWhiteSpace(char) method as a white-space character.