[GCU] Undefined variable in test_ecn_config_updates

[bingwang-ms]

If is_valid_platform_and_version returns False, we will see below exception

**Description**

        try:
            if is_valid_platform_and_version(duthost, "WRED_PROFILE", "ECN tuning"):
                output = apply_patch(duthost, json_data=json_patch, dest_file=tmpfile)
                expect_op_success(duthost, output)
                ensure_application_of_updated_config(duthost, configdb_field, ",".join(values))
            else:
>               expect_op_failure(output)
E               UnboundLocalError: local variable 'output' referenced before assignment

It's because the output variable is not defined.

Steps to reproduce the issue:
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Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

[bingwang-ms]

@isabelmsft Please check if the test case can run on Mellanox SPC3 platform, such as Mellanox-SN4600. Thanks

[isabelmsft]

Test case bug fixed in PR https://github.com/sonic-net/sonic-mgmt/pull/8652/files, requested review from scenario owner @neethajohn

[bingwang-ms]

Fixed.