added failsafe for auto update of jshell version

Author: Alathreon

.github/workflows/update-jshell-image-version.yaml

@@ -51,6 +51,12 @@ jobs:
           git config --global user.email "github-actions[bot]@users.noreply.github.com"
           
           branch_name="update-jshell-java-version-$latest_version"
+
+          if gh api repos/Together-Java/JShellPlaygroundBackend/branches/$branch_name >/dev/null 2>&1
+          then
+            return 0
+          fi
+
           git checkout -b "$branch_name"
           
           git add "$gradle_file"