[validation] Fix error on undefined optional numeric value

Providing no value for an optional numeric parameter results in
error "Parameter *** is invalid!"

This is caused by the validation function ignoring the 'required'
attribute when loading and checking input parameters.

This commit adds checks to determine whether the 'required' attri-
bute is defined and active before returning the error message.

References #570:

lib/validation.php

@@ -80,7 +80,7 @@ function validateData(&$data, $parameters){
 					break;
 				}
 
-				if(is_null($data[$name])) {
+				if(is_null($data[$name]) && isset($set[$name]['required']) && $set[$name]['required']) {
 					echo 'Parameter \'' . $name . '\' is invalid!' . PHP_EOL;
 					return false;
 				}