Add extra guard during db migration.

Relates to #13183

app/src/main/java/org/thoughtcrime/securesms/database/helpers/migration/V166_ThreadAndMessageForeignKeys.kt

@@ -66,10 +66,12 @@ object V166_ThreadAndMessageForeignKeys : SignalDatabaseMigration {
         Log.w(TAG, "There were $count threads for RecipientId::$recipientId. Merging.", true)
 
         val threads: List<ThreadInfo> = getThreadsByRecipientId(db, cursor.requireLong("thread_recipient_id"))
+        if (threads.size > 1) {
           mergeThreads(db, threads)
         }
       }
     }
+  }
 
   private fun getThreadsByRecipientId(db: SQLiteDatabase, recipientId: Long): List<ThreadInfo> {
     return db.rawQuery("SELECT _id, date FROM thread WHERE thread_recipient_id = ?".trimIndent(), recipientId).readToList { cursor ->