update beats: avoid failing if no files have changed (#6677) (#7181)

(cherry picked from commit 4e86049)

Co-authored-by: Victor Martinez <[email protected]>

.ci/scripts/update-beats.sh

@@ -9,9 +9,10 @@ make update-beats
 COMMIT_MESSAGE="Update to elastic/beats@$(go list -m -f {{.Version}} github.com/elastic/beats/... | cut -d- -f3)"
 
 git checkout -b "update-beats-$(date "+%Y%m%d%H%M%S")"
-git add go.mod go.sum NOTICE.txt \
+git add --ignore-errors go.mod go.sum NOTICE.txt \
 	.go-version docs/version.asciidoc \
 	docs/fields.asciidoc include/fields.go x-pack/apm-server/include/fields.go
+
 find . -maxdepth 2 -name Dockerfile -exec git add {} \;
 
 git diff --staged --quiet || git commit -m "$COMMIT_MESSAGE"