CI macOS: do not fail on sign if no keys

If no signing keys available, the expected behavior is rather to skip
signing instead of failing, which is unintended because of shell check
for unset variables (set -u).

.github/scripts/macOS/sign.sh

@@ -29,8 +29,9 @@ set -x
 
 APP=${1?Appname must be passed as a first argument}
 
-if [ -z "$KEY_CHAIN" ] || [ -z "$KEY_CHAIN_PASS" ] ||
-                { [ -z "$notarytool_credentials" ] && [ ! $sign_only ]; }; then
+if [ -z "${KEY_CHAIN-}" ] || [ -z "${KEY_CHAIN_PASS-}" ] ||
+                { [ -z "${notarytool_credentials-}" ] && [ ! $sign_only ]; }
+then
         echo "Could not find key to sign the application" 2>&1
         if [ "$GITHUB_REPOSITORY" = "CESNET/UltraGrid" ] && ! expr "$GITHUB_REF" : refs/pull >/dev/null; then
                 exit 1