feat(internal/kokoro): make publish_docs VERSION optional (#2979)

Author: tbpg

internal/kokoro/publish_docs.sh

@@ -19,6 +19,11 @@ set -eo pipefail
 # Display commands being run.
 set -x
 
+if [ -z "$MODULE" ] ; then
+    echo "Must set the MODULE environment variables"
+    exit 1
+fi
+
 python3 -m pip install --upgrade pip
 # Workaround for six 1.15 incompatibility issue.
 python3 -m pip install --use-feature=2020-resolver "gcp-docuploader<2019.0.0"
@@ -27,9 +32,8 @@ cd github/google-cloud-go/internal/godocfx
 go install
 cd -
 
-if [ -z "$MODULE" ] || [ -z "$VERSION" ] ; then
-    echo "Must set the MODULE and VERSION environment variables"
-    exit 1
+if [ -z "$VERSION" ] ; then
+    VERSION="latest"
 fi
 
 cd $(mktemp -d)