Fix button display when there is no panels (#8155)

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

web/ui/react-app/src/pages/graph/PanelList.tsx

@@ -105,7 +105,7 @@ export const PanelListContent: FC<PanelListProps> = ({
           enableMetricAutocomplete={enableMetricAutocomplete}
         />
       ))}
-      <Button className="mb-3" color="primary" onClick={addPanel}>
+      <Button className="d-block mb-3" color="primary" onClick={addPanel}>
         Add Panel
       </Button>
     </>