Do not allow registering a Model with an empty primary key

The Registry should either categorically allow registering a model
without a primary key or categorically not allow it. This fix implements
the latter. It does not use `empty()` because that would overshoot and
prevent the primary key from being `0` or `"0"`, which may not be
recommendable, but does not have to be forbidden in this context.

core-bundle/src/Resources/contao/library/Contao/Model/Registry.php

@@ -168,6 +168,13 @@ public function register(Model $objModel)
 		$strPk = $objModel->getPk();
 		$varPk = $objModel->$strPk;
 
+		if ($varPk === null || $varPk === '')
+		{
+			throw new \RuntimeException(
+				"Cannot register a model with a primary key that is null or empty string ($strTable::$strPk)"
+			);
+		}
+
 		// Another model object is pointing to the DB record already
 		if (isset($this->arrRegistry[$strTable][$varPk]))
 		{