Update tables.md

### What: 

Adding warning for null ```segment_id``` in Track event tables in cases where ID resolution has failed.

### Why:

Request for change in documentation following this issue: [Null segment_ids in tables](https://twilio-engineering.atlassian.net/browse/PS-403).

Author: ksaha873

src/unify/profiles-sync/tables.md

@@ -277,6 +277,9 @@ Segment's Identity Resolution has processed these events, which contain a `segme
 > info ""
 > To view and select individual track tables, edit your sync settings after you enable Profiles Sync, and wait for the initial sync to complete.
 
+> warning ""
+> These tables may have null segment_id in rare cases where ID resolution has failed, which could happen due to multiple reasons.
+
 
 
 ## Tables Segment materializes