Handle default/no value supplied case for ngMaxlength directive

If one adds ng-maxlength="" to an input field, the default value of 0 for max length is used which results in an incorrect error being added to the input field. This fix checks for a value of 0 for the maxlength. If one uses this directive and purposefully supplies a value of 0 for the directive, that would make absolutely no sense in any scenario I can think of. By using this as the 'default' value (or the value used if a blank value is passed for this directive), and checking for that value and returning a result of 'valid' if that value is supplied, corrects this issue. This is not a problem for the ng-minlength directive as having a default value of 0 for the minimum length is actually a 'no-op' as you can't have a length less than 0.