Merge pull request #23065 from TomG736/FixEmptyDisplayName

Fix null displayname crash as described in #21885

Author: MorrisJobke

lib/private/Group/Database.php

@@ -458,7 +458,11 @@ public function countDisabledInGroup(string $gid): int {
 
 	public function getDisplayName(string $gid): string {
 		if (isset($this->groupCache[$gid])) {
-			return $this->groupCache[$gid]['displayname'];
+			$displayName = $this->groupCache[$gid]['displayname'];
+
+			if (isset($displayName) && trim($displayName) !== '') {
+				return $displayName;
+			}
 		}
 
 		$this->fixDI();