[lib] Fix fatal error where empty code blocks crash
Summary:
There's a pretty fatal error where an empty code block will
trigger the throw Error statement. I suspect that it's because the
object for the message is { type: 'codeBlock', content: '\n' }.
It makes sense to just return the node if we have nothing to traverse within.
Test Plan:
Reproduced the message that causes the crash, alongside other more edge cases. Didn't experience a crash
Reviewers: ginsu, atul, tomek, ashoat
Reviewed By: ginsu, ashoat
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D5849