FORCE_INDEX=PRIMARY_KEY not supported by emulator

### Reproduce:
Create a query which contains `FROM MyTable@{FORCE_INDEX=PRIMARY_KEY}` statement

### Expected behaviour:
Query is executed (as it is using a regular Spanner instance)

### Actual outcome:
The following error is returned:
`com.google.cloud.spanner.SpannerException, with message: INVALID_ARGUMENT: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: The table does not have a secondary index called PRIMARY_KEY`