ci: harden unit test step and junit post

Jenkinsfile

@@ -95,26 +95,26 @@ spec:
     stage('Unit tests') {
       steps {
         container('tester') {
-          sh '''
-            set -euo pipefail
-            python -m pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
-            mkdir -p build
-            python -m ruff check ariadne --select C90,PLR
-            python -m slipcover \
-              --json \
-              --out "${COVERAGE_JSON}" \
-              --source ariadne \
-              --fail-under "${COVERAGE_MIN}" \
-              -m pytest -ra -vv --durations=20 --junitxml "${JUNIT_XML}"
-            python - <<'PY'
-            import json
-            with open("build/coverage.json", "r", encoding="utf-8") as handle:
-                payload = json.load(handle)
-            summary = payload.get("summary") or {}
-            percent = summary.get("percent_covered")
-            print(f"Coverage summary: {percent:.2f}%" if percent is not None else "Coverage summary unavailable")
-            PY
-          '''
+          sh(script: '''
+set -euo pipefail
+python -m pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
+mkdir -p build
+python -m ruff check ariadne --select C90,PLR
+python -m slipcover \
+  --json \
+  --out "${COVERAGE_JSON}" \
+  --source ariadne \
+  --fail-under "${COVERAGE_MIN}" \
+  -m pytest -ra -vv --durations=20 --junitxml "${JUNIT_XML}"
+python - <<'PY'
+import json
+with open("build/coverage.json", "r", encoding="utf-8") as handle:
+    payload = json.load(handle)
+summary = payload.get("summary") or {}
+percent = summary.get("percent_covered")
+print(f"Coverage summary: {percent:.2f}%" if percent is not None else "Coverage summary unavailable")
+PY
+'''.stripIndent())
         }
       }
     }
@@ -204,7 +204,15 @@ spec:
 
   post {
     always {
-      junit allowEmptyResults: true, testResults: 'build/junit.xml'
+      script {
+        if (fileExists('build/junit.xml')) {
+          try {
+            junit allowEmptyResults: true, testResults: 'build/junit.xml'
+          } catch (Throwable err) {
+            echo "junit step unavailable: ${err.class.simpleName}"
+          }
+        }
+      }
       archiveArtifacts artifacts: 'build/junit.xml,build/coverage.json', allowEmptyArchive: true, fingerprint: true
       script {
         def props = fileExists('build.env') ? readProperties(file: 'build.env') : [:]