[CommCoreModule] Return jsi::Value::undefined() in Get() else case

Summary: As a temporary solution, we were returning the string "unimplemented" when someone tried to get a property that didn't exist. Instead we should just return jsi::Value::undefined

Test Plan: Tried to access GRPCStreamHostObject.blah on the JS side and got undefined

Reviewers: ashoat, palys-swm, def-au1t, karol-bisztyga

Reviewed By: ashoat, palys-swm, karol-bisztyga

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3055