fail if there are no sync changes to commit instead of committing empty

[ajm188]

this prevents us from creating unnecessary empty commits and better communicates when there were no changes to sync.

running the script twice in a row results in the following on the second run:

+ git add
Nothing specified, nothing added.
hint: Maybe you wanted to say 'git add .'?
hint: Turn this message off by running
hint: "git config advice.addEmptyPathspec false"
+ git checkout -- .
+ git add content/en content/zh
++ git diff --cached --name-only
++ wc -l
+ '[' 0 -eq 0 ']'
+ echo 'No changes to cobradocs detected'
No changes to cobradocs detected
+ exit 1

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