[ci] Fix escaping for gen_versions_map.sh script (#836)

Backport of #836

fixes errors like this:

```
make: Entering directory '/home/runner/work/cozystack/cozystack/packages/apps'
find . -maxdepth 2 -name Chart.yaml  | awk -F/ '{print $2}' | while read i; do sed -i "s/^name: .*/name: $i/" "$i/Chart.yaml"; done
../../hack/gen_versions_map.sh
../../hack/gen_versions_map.sh: 34: [: !=: unexpected operator
fatal: Needed a single revision
make: *** [Makefile:17: gen-versions-map] Error 128
make: Leaving directory '/home/runner/work/cozystack/cozystack/packages/apps'
```
https://github.com/cozystack/cozystack/actions/runs/14591720553/job/40928276862?pr=835

Improved reliability of version generation by handling empty or
special values safely in the process.

(cherry picked from commit 7a9a1fcba4)

Signed-off-by: Nick Volynkin <nick.volynkin@gmail.com>

hack/gen_versions_map.sh

@@ -30,7 +30,7 @@ resolved_miss_map=$(
     fi
 
     # if commit is not HEAD, check if it's valid
-    if [ $commit != "HEAD" ]; then
+    if [ "x$commit" != "xHEAD" ]; then
       if [ $(git show "${commit}:./${chart}/Chart.yaml" 2>/dev/null | awk '$1 == "version:" {print $2}') != "${version}" ]; then
         echo "Commit $commit for $chart $version is not valid" >&2
         exit 1