Make passing NULL for host not crash

Makefile

src/core/channel/http_server_filter.c

src/core/surface/channel.c

src/core/surface/server.c

test/core/end2end/gen_build_json.py

test/core/end2end/tests/default_host.c

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json

vsprojects/Grpc.mak