CI: Try to commit changes only if we optimized some svgs

[why]
When so svg files could be optimized we still try to commit the
'changes'. There are no changes - so nothing is committed (empty commits
are avoided).

But the workflow run still shows the 'commit back to repo' step,
although we know beforehand that it will not commit anything.

[how]
Technically that is no problem and the behavior is unchanged, but we can
just skip the commit step if we know there can not be anything to
commit...

It just looks nicer :-}

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

.github/workflows/packsvgs.yml

@@ -21,11 +21,16 @@ jobs:
           sudo apt install python3-fontforge inkscape texlive-binaries dc -y -q
 
       - name: Simplify the SVGs
+        id: optimize
         run: |
           cd bin/scripts
-          ./optimize-original-source.sh doit
+          ./optimize-original-source.sh doit | tee optlog.txt
+          changenum=$(tail -n 1 optlog.txt | sed -E 's/[^0-9]*([0-9]+).*/\1/')
+          echo "Script claims number of changed files: ${changenum}"
+          echo "::set-output name=num_of_files::${changenum}"
 
       - name: Commit simplified SVGs back to repo
+        if: steps.optimize.outputs.num_of_files > 0
         uses: EndBug/add-and-commit@v9
         with:
           fetch: false