_webform_validate_check_false() should count empty strings as false

This bug applies to both versions six and seven, and I discovered it while trying to make Webform Structured Text work with webform_validate. The issue is that webform_validate flattens any item that has a value array into a string separated by commas. The function callback to determine if one of those value array elements is 'there' or not only looks for exactly equaling FALSE or zero (0), but does not include empty strings. So, when attempting to put a webform_structured_text element into a some-of-several type validation rule, what comes back to the validation function is a string of commas instead of an empty string, and so the validation fail is not fired even though the webform_structured_text component was indeed empty.

I think in addition to the test of $var in the check-false function for FALSE and 0, it should also provide a filter for empty strings. It's a simple addition of && $var !== '' to the function, thus:

function _webform_validation_check_false($var) {
  return $var !== FALSE && $var !== 0 && $var !== '';  
}

Once this is included, I can add the hook into Webform Structured Text so that it can participate in Webform Validation validations.

Shawn