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](https://github.com/envoyproxy/envoy/blob/main/test/extensions/filters/http/common/fuzz/filter_fuzz_test.cc#L29) to force the test to pick ext_proc and reproduce the failure.