fix: Add shopt -s nullglob to dependencies script (#1130)

This prevents processing the unexpanded glob "**/.flattened-pom.xml" in repos which do not flatten any POMs.

Author: dpcollins-google

synthtool/gcp/templates/java_library/.kokoro/dependencies.sh

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 set -eo pipefail
+shopt -s nullglob
 
 ## Get the directory of the build script
 scriptDir=$(realpath $(dirname "${BASH_SOURCE[0]}"))