refactor(ci): Generate changelog only when it does not exist

.github/workflows/editor-changelog.yaml

@@ -25,13 +25,24 @@ jobs:
           CURRENT_VERSION=$(node -p "require('./packages/editor/package.json').version")
           echo "CURRENT_VERSION=$CURRENT_VERSION" >> $GITHUB_ENV
 
+      - name: Check if changelog for this version already exists
+        id: check_existing_changelog
+        run: |
+          if [ -f packages/editor/CHANGELOG.md ] && grep -q "## Changelog for version $CURRENT_VERSION" packages/editor/CHANGELOG.md; then
+            echo "changelog_exists=true" >> $GITHUB_ENV
+          else
+            echo "changelog_exists=false" >> $GITHUB_ENV
+          fi
+
       - name: Get latest published version from npm
+        if: env.changelog_exists == 'false'
         id: get_npm_version
         run: |
           NPM_VERSION=$(npm view @serlo/editor version)
           echo "NPM_VERSION=$NPM_VERSION" >> $GITHUB_ENV
 
       - name: Check if version has changed
+        if: env.changelog_exists == 'false'
         id: check_version
         run: |
           if [ "$CURRENT_VERSION" != "$NPM_VERSION" ]; then