ci: fix the remaining shellcheck findings and enable the check (#12367)

.github/workflows/prepare-release.yml

@@ -77,7 +77,7 @@ jobs:
 
           echo "Prowler version: $PROWLER_VERSION"
           echo "Branch name: $BRANCH_NAME"
-          echo "Is minor release: $([ $PATCH_VERSION -eq 0 ] && echo 'true' || echo 'false')"
+          echo "Is minor release: $([ "$PATCH_VERSION" -eq 0 ] && echo 'true' || echo 'false')"
         else
           echo "Invalid version syntax: '$PROWLER_VERSION' (must be N.N.N)" >&2
           exit 1
@@ -107,7 +107,8 @@ jobs:
           if [ -f "$changelog_file" ]; then
             # Extract version that matches this Prowler release
             # Format: ## [version] (Prowler X.Y.Z) or ## [vversion] (Prowler vX.Y.Z)
-            local version=$(grep '^## \[' "$changelog_file" | grep "(Prowler v\?${prowler_version})" | head -1 | sed 's/^## \[\(.*\)\].*/\1/' | sed 's/^v//' | tr -d '[:space:]')
+            local version
+            version=$(grep '^## \[' "$changelog_file" | grep "(Prowler v\?${prowler_version})" | head -1 | sed 's/^## \[\(.*\)\].*/\1/' | sed 's/^v//' | tr -d '[:space:]')
             echo "$version"
           else
             echo ""
@@ -226,7 +227,7 @@ jobs:
         fi
 
         # Combine changelogs in order: UI, API, SDK, MCP
-        > combined_changelog.md
+        : > combined_changelog.md
 
         if [ "$HAS_UI_CHANGES" = "true" ] && [ -s "ui_changelog.md" ]; then
           echo "## UI" >> combined_changelog.md