Restore support for access control filenames without a group

Regression from commit b15ef71
of release 1.1.0, detailed analysis online at
#540 (comment)

Author: hartwork

src/Daemon/Daemon.cpp

@@ -451,7 +451,11 @@ namespace usbguard
     const std::string user = basename.substr(0, ug_separator);
     const std::string group = has_group ? basename.substr(ug_separator + 1) : std::string();
     checkIPCAccessControlName(user);
-    checkIPCAccessControlName(group);
+
+    if (! group.empty()) {
+      checkIPCAccessControlName(group);
+    }
+
     *ptr_user = user;
     *ptr_group = group;
   }