[TS] Fix nullability (#2702)

`LGraphCanvas.graph` may explicity set to null by litegraph.

src/composables/useCoreCommands.ts

@@ -337,7 +337,7 @@ export function useCoreCommands(): ComfyCommand[] {
           'Comfy.GroupSelectedNodes.Padding'
         )
         group.resizeTo(canvas.selectedItems, padding)
-        canvas.graph.add(group)
+        canvas.graph?.add(group)
         useTitleEditorStore().titleEditorTarget = group
       }
     },

src/composables/usePaste.ts

@@ -66,8 +66,8 @@ export const usePaste = () => {
           const newNode = LiteGraph.createNode('LoadImage')
           // @ts-expect-error array to Float32Array
           newNode.pos = [...canvas.graph_mouse]
-          imageNode = graph.add(newNode) ?? null
-          graph.change()
+          imageNode = graph?.add(newNode) ?? null
+          graph?.change()
         }
         pasteItemOnNode(items, imageNode)
         return