fix(ui): fix erroneous vae model display

`react-select` has some weird behaviour where if the value is `undefined`, it shows the last-selected value instead of nothing. Must fall back to `null`

Author: psychedelicious

invokeai/frontend/web/src/common/components/InvSelect/useGroupedModelInvSelect.ts

@@ -65,7 +65,7 @@ export const useGroupedModelInvSelect = <T extends AnyModelConfigEntity>(
         .flatMap((o) => o.options)
         .find((m) =>
           selectedModel ? m.value === getModelId(selectedModel) : false
-        ),
+        ) ?? null,
     [options, selectedModel]
   );