Correct check against empty value in MD_STD_IN

MD_STD_IN.php

@@ -55,7 +55,7 @@ final public static function sanitize_id_or_zero($input):int {
             ]
         );
 
-        if ($input === null) {
+        if ($input === false) {
             throw new MDpageParameterNotNumericException("Value is not numeric.");
         }