Emit the correct instruction for add_row_with_key(none)

CHANGELOG.md

@@ -9,7 +9,9 @@
 * Compacting a realm into an encrypted file could take a really long time. The process is now optimized by adjusting the write
   buffer size relative to the used space in the realm.
   ([#2754](https://github.com/realm/realm-sync/issues/2754))
-
+* Creating an object after creating an object with the int primary key of "null" would hit an assertion failure.
+  ([#3227](https://github.com/realm/realm-core/pull/3227)).
+
 ### Breaking changes
 * None.
 

src/realm/table.cpp

@@ -2251,7 +2251,7 @@ size_t Table::add_row_with_key(size_t key_col_ndx, util::Optional<int64_t> key)
             repl->add_row_with_key(this, row_ndx, prior_num_rows, key_col_ndx, *key); // Throws
         else {
             repl->insert_empty_rows(this, row_ndx, 1, prior_num_rows); // Throws
-            repl->set_null(this, key_col_ndx, row_ndx);
+            repl->set_null(this, key_col_ndx, row_ndx, _impl::instr_SetUnique);
         }
     }