Convenience method for trimming, truncating and nullifying of string.

• If string is empty or containing whitespace only, then the method returns null.
• Otherwise the string is trimmed
• If parameter maxLength is provided then the string will be shortened if necessary.

string value = ...;

var trimmedOrNull = value.TrimOrNullify();

var trimmedAndShortenedOrNull = value.TrimOrNullify(maxLength: 10);