Fix optional contentType issue

[JoshLove-msft]

As described in Azure/azure-sdk-for-net#55300, but the issue belongs in this repo.

[ArcturusZhang]

I think this is not fixed.
The fix introduced an extra check to see if it is a content type header, by checking if the sdkProperty is not undefined, but in our code, when the fromSdkType is called converting the method parameters (for instance here), the sdkProperty argument is never passed in. Therefore the fix is not actually running.