Loosen the the test for the date format (Bug #14130).

Theoretically it might have been intended to explicitly pass an empty string (as opposed to a null value) as the date format. But this doesn't make sense because that would generate a form field that never has a result value. Beside that we already *do* catch an empty value, even if it's a strict null check.

framework/Form/lib/Horde/Form/Type.php

@@ -3021,7 +3021,7 @@ function _validateAndFormat($value, &$var)
             return $var->getDefault();
         } else {
             $date = $this->getDateOb($value);
-            if ($this->_format_in === null) {
+            if (!strlen($this->_format_in)) {
                 return $date->timestamp();
             } else {
                 return $date->strftime($this->_format_in);