json: Fix issue converting null yaml into json

[snowp]

When envoy is launched with an empty yaml file through -c it blows up with:

2018-03-02_15:39:44.80673 [2018-03-02 15:39:44.806][1][critical][assert] external/envoy/source/common/json/json_loader.cc:311] panic: not reached
2018-03-02_15:39:44.80675 [2018-03-02 15:39:44.806][1][critical][backtrace] bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:11
4] Caught Aborted, suspect faulting address 0x4e3800000001
2018-03-02_15:39:44.80698 [2018-03-02 15:39:44.806][1][critical][backtrace] bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:90
] Backtrace obj</lib/x86_64-linux-gnu/libc.so.6> thr<0> (use tools/stack_decode.py):

This happens because the yaml loader correctly reads the file as a null value but converting this into a rapidjson object fails because the Null type isn't handled. This PR simply adds a switch case that handles this case by returning a null json object.

Risk Level: Low, it makes the json generator more permissive

Testing:
Added unit test that verifies that an empty string can be parsed as yaml and converted to a json string. Also verified that envoy doesn't crash when given an empty yaml file:

➜  envoy git:(empty-yaml) ✗ bazel-bin/source/exe/envoy-static -c empty.yaml
[2018-03-02 23:11:52.558][2664388][info][main] source/server/server.cc:178] initializing epoch 0 (hot restart version=disabled)
[2018-03-02 23:11:52.572][2664388][critical][main] source/server/server.cc:71] error initializing configuration 'empty.yaml': JSON at lines 0-0 does not conform to schema.
 Invalid schema: #
 Schema violation: type
 Offending document key: #
[2018-03-02 23:11:52.572][2664388][info][main] source/server/server.cc:392] exiting

[mattklein123]

Awesome, thanks. Small nit if you feel like it.

[mattklein123]

nit if you feel like it: I would remove brace at 449/452. They are not needed as this is the only thing that happens in this test.

[snowp]

done!

[htuch]

Thanks!