Type error fix for empty string on 'validate-date'

fixes #1030

When input value is a empty string of spaces, date-validator returns
`null`, and `formatted = date.format(format);` at [L462](https://github.com/mootools/mootools-more/blob/master/Source/Forms/Form.Validator.js#L462) gives a type error (http://jsfiddle.net/8YH3w/). 

    TypeError: Cannot read property 'format' of null

This commit returns false on such cases and corrects indent at lines [458-464](https://github.com/mootools/mootools-more/blob/master/Source/Forms/Form.Validator.js#L458-L464).