[nit] Avoid dispatching to parent when not needed in c_glib implementation (#3149)

As titled. This avoids dispatching calls up to the parent implementation if there isn't one, saving a hop.

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I tested this by:

* Building the compiler
* Building the c_glib tutorial server and client
* Verifying that the client/server interaction works as previously expected
* Inspecting the generated code and noticing shared_service.c does not have the code to dispatch to its parent anymore, while the calculator.c implementation does.