fix: Add shopt -s nullglob to dependencies script (#1130) (#517)

This prevents processing the unexpanded glob "**/.flattened-pom.xml" in repos which do not flatten any POMs.
Source-Link: googleapis/synthtool@87254ac
Post-Processor: gcr.io/repo-automation-bots/owlbot-java:latest@sha256:a9b83a276c82987d284b733713fe86a3c8a2cc457933767eeb2688b882e54c6a

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-java:latest
-  digest: sha256:117d9f1bef3342ea256419068dfaf65e5b2d126534ef0773c25ce71b8474f346
+  digest: sha256:a9b83a276c82987d284b733713fe86a3c8a2cc457933767eeb2688b882e54c6a

.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]}"))