Protect against unknown GV2 UUIDs.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/util/UuidUtil.java

@@ -81,4 +81,19 @@ public final class UuidUtil {
 
     return result;
   }
+
+  /**
+   * Keep only UUIDs that are not the {@link #UNKNOWN_UUID}.
+   */
+  public static List<UUID> filterKnown(Collection<UUID> uuids) {
+    ArrayList<UUID> result = new ArrayList<>(uuids.size());
+
+    for (UUID uuid : uuids) {
+      if (!UNKNOWN_UUID.equals(uuid)) {
+        result.add(uuid);
+      }
+    }
+
+    return result;
+  }
 }