fix(build): allow git add to fail if there aren't new docs

[silvolu]

No description provided.

[stephenplusplus]

Cool 👍

[dhermes]

I noticed this looked similar to something I had done so figured I'd take a peek.

Notice committing nothing also has a non-zero exit-status:

$ echo $?
0
$ git commit -m "Hey"
...
nothing added to commit but untracked files present
$ echo $?
1

You could check the exit status of git add and then only commit and push (with set -e) if the exit status was 0. This is kind of done in gcloud-python-wheels.