import error for multipart when there is only anonymous body type

[msyyc]

• tsp: https://github.com/Azure/azure-rest-api-specs-pr/blob/7e513eea2c42a176ee8d89c9fde8625d6dc58ad6/specification/orbitalplanetarycomputer/Orbital.Spatio/routes.stac.tsp#L355

• the generated sdk code: https://github.com/Azure/azure-sdk-for-python/blob/2eec36253fb53d4b804c33aa128fca7bda6af3f5/sdk/spatio/spatio/spatio/aio/operations/_operations.py#L1123-L1126

We can see that _model_base is used without import.