[Buildkite] Fix unbound variable for docs bypass (#711)

.buildkite/deployment.sh

@@ -7,6 +7,8 @@ if [[ $BUILDKITE_TAG == "" ]]; then
   else
     CI_DOCS_BYPASS=$(git diff --name-only `git merge-base --fork-point origin/master` | sed -rn '/^docs\/.*/!{q1}' && echo true || echo false)
   fi
+else
+  CI_DOCS_BYPASS="false"
 fi
 
 cat << EOF

.buildkite/pipeline.sh

@@ -11,6 +11,8 @@ if [[ $BUILDKITE_TAG == "" ]]; then
   if [[ $CI_DOCS_BYPASS == "true" ]]; then
   cat .buildkite/annotations/documentation | buildkite-agent annotate --style "info" --context "ctx-info"
   fi
+else
+  CI_DOCS_BYPASS="false"
 fi
 
 cat << EOF