When typespec-autorest emitter includes default values in PATCH APIs

If all of the properties of a model are optional (`?`) and using `ArmResourcePatchSync`/`Async`, the emitter will not define and emit a new Update model to represent the PATCH operation. This results in what I think is improper spec (or, at least, [azure-openapi-validator](https://github.com/Azure/azure-openapi-validator) complains about it).

[Playground Link](https://cadlplayground.z22.web.core.windows.net/cadl-azure/?options=%7B%22linterRuleSet%22%3A%7B%22extends%22%3A%5B%22%40azure-tools%2Ftypespec-azure-rulesets%2Fresource-manager%22%5D%7D%7D&c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7CtIZcmVzdNUZdmVyc2lvbmluZ8wfYXp1cmUtdG9vbHMvyCstxhVjb3Jl3yvIK3Jlc291cmNlLW1hbmFnZXIiOwoKdXNpbmcgVHlwZVNwZWMuSHR0cDvQFVJlc3TRFVbpAKnIG0HEZS5Db3JlzhJSx3dNxnY7CgovKiogQ29udG9zbyDIHiBQcm92aWRlciDmAJ5tZW50IEFQSS4gKi8KQGFybcggTmFtZXNwYWNlCkBzZXJ2aWNlKHsKICB0aXRsZTogIsdYyC5IdWJDbGllbnQiLAp9KQpA5wFgZWQo5wDBcykKbshSIE1pY3Jvc29mdC7SR%2B8AuEFQSSDHTXPkAKNlbnVtIMhUIOQAksQuMjAyMS0xMC0wMS1wcmV2aWV3yDXENCAgQHVzZURlcGVuZGVuY3ko9QEx6AFrcy52MV8wX1DGSF8xKcRAYXJtQ29tbW9u5AGZc8cq10jLKctUNcRIYPIAqWAsCn3mAPNB6AD16wEOIOgCTeQAyG1vZGVsIEVtcGxveWVlIGlzIFRyYWNrZWToAII8yBxQcm9wZXJ0aWVzPuUBJC4u6QCm5AHYUGFyYW1ldGVyyTE%2BO8Ym5gDEZFPmAfJJZGVudGl0ecdPeTvoAKvINyBwyWnyAKHHNsQc6AGoQWdlIG9mIGXIP%2BYBnXZpc2liaWxpdHkoImNyZWF0ZSIsICJyZWFkIuQBNWFnZT86IGludDMyID0gMjE7CsdOQ2l0ed9P009jaXR5Pzogc3Ry5AM8PSAiUmVkbW9uZOQDp8ZYVGhlIHN0YXR1c8RedGhlIGxhc3Qg5ADOYXRpb27lAyzPaclfcOQBvecDs1N0YXTkALzlAdHMFOkBScRrzDLFeOUBJGHpAfvFb0Bscm%2FEO3VzCnVu5ALY0VTlAWXmANYs7ADKyEcg5gEMIHJlcXVlc3QgaGFzIGJlZW4gYWNjZXB0ZWTEZyAgQccOOiAiyAsi1lBpxEDkALTpAMHIROwAnDogIswP2kx1cGRhdMRPxUNVxw46ICLIC8o76QTN6QDE5gDcZOcBmlN1Y2NlZWTlAMXJDNM%2FxTbkAU1mYWlsyT5GxQ06ICLGCdw4d2FzIGNhbmNlyj5Dxw%2FkBSTHC%2F8BQCBkZWxl6QGARMQN5gD5yAvkBVQKCmludGVyZuQFP0%2FoAolzIGV4dGVuZHP2BgMuyyl7fQrlBd%2FII8oby1noA7rmA7HlAZHlAKdByjhQYXRjaFN5bmPpBDss8wPxPuQC4Q%3D%3D&e=%40azure-tools%2Ftypespec-autorest)

In the above playground, the patch expects the `EmployeeUpdate` which contains `EmployeeProperties` and the `ManagedServiceIdentityUpdate`. However, the properties of `EmployeeProperties` are all create/read-only properties. Removing the nullability of one of the properties--say, age--will instead prompt the emittance of `EmployeeUpdate` containing `EmployeePropertiesUpdate`, which in turn is defined with no properties (as nothing from `EmployeeProperties` should be able to be updated.

Not sure if this improper definition by us, though. We're running into [RPC-Patch-V1-02](https://github.com/Azure/azure-openapi-validator/blob/main/docs/un-supported-patch-properties.md) and [RPC-Patch-V1-10](https://github.com/Azure/azure-openapi-validator/blob/main/docs/patch-body-parameters-schema.md) because of this.

Start with the linked Playground reproduction and trace the ArmResourcePatchSync/Async emitter behavior for models whose properties are all optional or create/read-only. Compare the generated PATCH schemas with RPC-Patch-V1-02 and RPC-Patch-V1-10, and consider the issue done when the reproduction no longer triggers those validator complaints.

Written by the indexing model from the issue text.