aws ecs deploy: --cluster "" (empty string) is not treated as unspecified, unlike omitting --cluster

### Describe the bug

`ECSClient.get_service_details()`, used by `aws ecs deploy`, has a broken conditional for defaulting the `--cluster` value:

```python
def get_service_details(self):
cluster = self._args.cluster

if cluster is None or '':
cluster = 'default'
```

Due to Python operator precedence, this parses as `if (cluster is None) or (''):`. The empty string literal `''` is always falsy, so it contributes nothing to the condition — the `or ''` is dead code. The check is silently equivalent to just `if cluster is None:`.

The command's own help text for `--cluster` says:

> If you do not specify a cluster, the "default" cluster is assumed.

That's true when `--cluster` is omitted entirely (`argparse` leaves it as `None`). But if a caller passes `--cluster ""` — for example a CI/CD pipeline doing `aws ecs deploy --cluster "$CLUSTER_NAME" ...` where `$CLUSTER_NAME` is unset or empty — `cluster` is an empty string, not `None`, so the fallback to `"default"` is skipped and the empty string is sent straight through to `DescribeServices`.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

`--cluster ""` should be treated the same as omitting `--cluster`, and fall back to the `"default"` cluster, matching the documented behavior.

### Current Behavior

`describe_services` is called with `cluster=''` instead of `cluster='default'`, which does not match the documented "default cluster is assumed" fallback and will fail cluster/service resolution against the ECS API instead of deploying to the default cluster.

Demonstrated directly against `ECSClient.get_service_details()`:

```python
>>> client._args.cluster = ''
>>> client.get_service_details()
# describe_services is invoked with cluster='' instead of cluster='default'
```

### Reproduction Steps

```console
$ CLUSTER_NAME=
$ aws ecs deploy --service my-service --cluster "$CLUSTER_NAME" \
--task-definition file://task-def.json \
--codedeploy-appspec file://appspec.yaml
```

Because `$CLUSTER_NAME` is empty, this passes `--cluster ""`, and the command does not fall back to the `default` cluster the way omitting `--cluster` entirely would.

### Possible Solution

Replace the broken condition with `if not cluster:`, which correctly treats both `None` and `''` as "not specified" and falls back to `'default'` in both cases. I have a PR ready with the fix and regression tests (there was previously no test coverage at all for the cluster-defaulting behavior in `get_service_details`).

### CLI version used

aws-cli/2.36.23 (reproduced on `v2` at 4c331fc)

### Environment details (OS name and version, etc.)

macOS 15 (Darwin 25.2.0), Python 3.12 — not OS-specific, this is a pure logic bug.

Start at ECSClient.get_service_details and trace how the --cluster argument is passed to describe_services. Add regression coverage for omitted and empty --cluster values, then run the relevant AWS CLI tests to verify both cases use the documented default cluster behavior.

Written by the indexing model from the issue text.