Fix GraphViz extension support (#803)

Author: JeroenDeDauw

src/Graph/GraphPrinter.php

@@ -114,7 +114,7 @@ public function hasMissingDependency() {
 		$registry = \ExtensionRegistry::getInstance();
 		return (
 			// <graphviz> can be provided by Diagrams.
-			!$registry->isLoaded( 'Diagrams' ) ||
+			!$registry->isLoaded( 'Diagrams' ) &&
 			!class_exists( 'GraphViz' ) && !class_exists( '\\MediaWiki\\Extension\\GraphViz\\GraphViz' )
 		) && !(
 			// <graphviz can also be added by External Data in Tag emulation mode.