watchman: w_string_to_json now returns json_null for null inputs
watchman: w_string_to_json now returns json_null for null inputs

Summary:
Previously, this function would return nullptr and this
was the source of occasional crashes in various debug dumps and
diagnostic commands.

This commit changes the function to return a json_null value instead.

Reviewed By: fanzeyi

Differential Revision: D23964789

fbshipit-source-id: c489ba4b6abb24d23aa8214d28ea6a47b906bb5a

facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman