[ci] Deploy only when there is something to push

Apparently the previous version of gtk-doc was doing weird changes like https://github.com/harfbuzz/harfbuzz.github.io/commit/1b40da99bbd1985d060d273ef37f1ccad16913c4 and https://github.com/harfbuzz/harfbuzz.github.io/commit/d1d9bff14c805d1bcbd2446a4c22e633e8d365cf
in order to make every to have something for pushing and
now that we don't have such changes it will fail.

Alternatively we can go for `git commit --allow-empty`, guess better to
pollute history of https://github.com/harfbuzz/harfbuzz.github.io however.
diff --git a/.ci/deploy-docs.sh b/.ci/deploy-docs.sh
index 2209bf7..854ca23 100755
--- a/.ci/deploy-docs.sh
+++ b/.ci/deploy-docs.sh
@@ -27,5 +27,8 @@
 
 touch .
 git add -A .
-git commit -m "Rebuild docs for https://github.com/harfbuzz/harfbuzz/commit/$REVISION"
-git push -q upstream HEAD:master
+
+if [[ $(git status -s) ]]; then
+  git commit -m "Rebuild docs for https://github.com/harfbuzz/harfbuzz/commit/$REVISION"
+  git push -q upstream HEAD:master
+fi