Inconsistent handling of null and boxed null parameter

[axunonb]

Obviously there are cases where exceptions are thrown even when ErrorAction.ThrowError is not set.

Smart.Default.Settings.FormatErrorAction = ErrorAction.MaintainTokens;
Smart.Default.Settings.ParseErrorAction = ErrorAction.MaintainTokens;
var result = Smart.Format("{dummy}", null); // always throws an exception - interpreted as IEnumerable
result = Smart.Format("{dummy}", (object) null); // works

Closes axuno/MailMergeLib#14

[axunonb]

The current handling of a null parameter is inconsistent:

object boxedNull = null;
if (boxedNull != null) throw new Exception("This never happens!");
Smart.Format("{dummy}", null); // throws exception
Smart.Format("{dummy}", boxedNull); // works as expected

Will be fixed in the way that null will be handled as a valid argument.