[LorisForm] Fixing the loris form '0' emptying issue (#2781)

$this->form->addRule("value", "Required", 'required'); sets the 'required fail' when the user submits a value of "0", because empty($value) is true for 0.

This updates the code to explicitly check for the empty string to detect no input, and avoids PHP's empty() function for validation.

php/libraries/LorisForm.class.inc

@@ -1180,7 +1180,7 @@ class LorisForm
 
                 if (isset($el['required'])
                     && $el['required'] === true
-                    && empty($value)
+                    && $value === ""
                 ) {
                     $errors[$el['name']] = "required fail";
                 }