fix: add missing nil check on create group with avatar

mentioned in commit ec592037

mentioned in merge request gitlab-org/cli!2107 (merged)

mentioned in merge request gitlab-org/release-tools!4241 (merged)

mentioned in merge request fforster/gitlab-mcp!88 (merged)

mentioned in merge request schober-server/kubernetes/images/gitlab-token-refresher!14 (merged)

mentioned in merge request gitlab-com/gl-infra/autolabels!58 (merged)

mentioned in merge request gitlab-com/gl-infra/ringctl!309 (merged)

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!1032 (merged)

mentioned in merge request gitlab-org/terraform-provider-gitlab!2545 (closed)

mentioned in merge request gitlab-com/gl-infra/woodhouse!643 (merged)

mentioned in merge request gitlab-org/cli!2147 (closed)

mentioned in merge request gitlab-org/container-registry!2355 (merged)

mentioned in merge request sue445/tanuki_reminder!252 (merged)

mentioned in merge request sue445/create-merge-request!137 (merged)

mentioned in merge request fforster/epic-update!12 (merged)

mentioned in merge request gitlab-com/gl-infra/oncall-robot-assistant!104