chore(java): ignore return code 28 in README autosynth job (#255)

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Synth log will be available here:
https://source.cloud.google.com/results/invocations/01cd0979-7ef1-4bfe-ad90-981b903afc4a/targets

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Source-Link: googleapis/synthtool@7db8a6c

Author: yoshi-automation

.kokoro/readme.sh

@@ -28,9 +28,18 @@ echo "https://${GITHUB_TOKEN}:@github.com" >> ~/.git-credentials
 git config --global credential.helper 'store --file ~/.git-credentials'
 
 python3.6 -m pip install git+https://github.com/googleapis/synthtool.git#egg=gcp-synthtool
+
+set +e
 python3.6 -m autosynth.synth \
     --repository=googleapis/java-dns \
     --synth-file-name=.github/readme/synth.py \
     --metadata-path=.github/readme/synth.metadata \
     --pr-title="chore: regenerate README" \
-    --branch-suffix="readme"
+    --branch-suffix="readme"
+
+# autosynth returns 28 to signal there are no changes
+RETURN_CODE=$?
+if [[ ${RETURN_CODE} -ne 0 && ${RETURN_CODE} -ne 28 ]]
+then
+    exit ${RETURN_CODE}
+fi

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-dns.git",
-        "sha": "fa9a8263528dac7943848a8290aee2d889431e3c"
+        "sha": "95eddf3b1688d6fd3a1b070bc3757fb0dcff554d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "c7824ea48ff6d4d42dfae0849aec8a85acd90bd9"
+        "sha": "7db8a6c5ffb12a6e4c2f799c18f00f7f3d60e279"
       }
     }
   ],