filter_fuzz_test fails for ext_proc filter

Description: filter_fuzz_test picks http filters by random to fuzz. The test fails when it picks the ext_proc filter, as the filter is instantiated with a mock Grpc::AsyncClientManager, which returns a null grpc client. The test seg faults when invoking a method on the null grpc client.

Repro steps: Replace seed % filter_names.size() with 5 here to force the test to pick ext_proc and reproduce the failure.