fix(ui): do not allow model add when no location is provided

Author: psychedelicious

invokeai/frontend/web/src/features/modelManagerV2/subpanels/AddModelPanel/InstallModelForm.tsx

@@ -72,7 +72,7 @@ export const InstallModelForm = () => {
           </FormControl>
           <Button
             onClick={handleSubmit(onSubmit)}
-            isDisabled={!formState.isDirty}
+            isDisabled={!formState.dirtyFields.location}
             isLoading={isLoading}
             type="submit"
             size="sm"