[tcgc]filter out empty namespaces in TCGC output

[live1206]

Clear and concise description of the problem

Namespaces with no operations and no sub-clients should be filtered out.
Interfaces without operations should still be included because in that case the customer is explicitly defining the interface(garbage in, garbage out),
whereas for namespaces they may be defining a models-only namespace in which case a client is unnecessary.

Checklist

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[live1206]

shifted from microsoft/typespec#5624 (comment)