markExhausted throws IllegalArgumentException for template-based instance configurations, breaking zone fallback

### Jenkins and plugins versions report

- Jenkins: 2.520+ (observed on current LTS line)
- google-compute-engine: 4.815.v9a_de97053e3a_ (bug introduced with the zone-fallback feature, #555)

### What Operating System are you using (both controller, and any agents involved in the problem)?

Controller: Linux (k8s). Agents: GCE VMs, Debian 13.

### Reproduction steps

1. Configure a GCE cloud with an **instance-template-based** instance configuration (template set, machine type field left empty — the normal state for template-based configs, since `machineType` is only used when no template is set).
2. Have GCE return a capacity error on instance insert (`ZONE_RESOURCE_POOL_EXHAUSTED`, reason `stockout`).
3. The launcher detects the capacity error and calls `config.markExhausted(zone)`.

### Expected Results

The zone is marked exhausted and the next provisioning cycle uses a fallback zone (the new #555 behavior).

### Actual Results

`markExhausted` throws a bare `IllegalArgumentException` from `ClientUtil.nameFromSelfLink("")` while formatting its **log message**, because template-based configurations have an empty `machineType`:

```
java.lang.IllegalArgumentException
at com.google.common.base.Preconditions.checkArgument(Preconditions.java:127)
at com.google.cloud.graphite.platforms.plugin.client.util.ClientUtil.nameFromSelfLink(ClientUtil.java:84)
at com.google.jenkins.plugins.computeengine.InstanceConfiguration.markExhausted(InstanceConfiguration.java:501)
at com.google.jenkins.plugins.computeengine.ComputeEngineComputerLauncher.launch(ComputeEngineComputerLauncher.java:209)
at hudson.slaves.SlaveComputer.lambda$_connect$0(SlaveComputer.java:298)
```

Jenkins surfaces it as "ERROR: Unexpected error in launching an agent. This is probably a bug in Jenkins" on the node's launch log, repeatedly (the retention strategy retries `connect()` forever).

Consequences:
- the exception propagates out of `launch()` **before** `terminateNode()` runs, so the dead node object is never cleaned up and the retention strategy relaunches it in a loop;
- the zone is never marked exhausted, so the fallback-zone feature never engages and the provisioner keeps hammering the stocked-out zone.

We hit this in production during a real c3d/c4d stockout in us-central1-a; every template-based config crashed the same way.

### Anything else?

Workaround we used: populate the (otherwise unused) machine type field on template-based configurations — `instance()` only reads `machineType` in the no-template branch, so this is safe and stops the crash. After that, the #555 fallback logic worked exactly as designed under a subsequent real stockout.

### Are you interested in contributing a fix?

Yes — PR incoming (guard the log message for empty `machineType`, fall back to the template name, plus a regression test).

Start in InstanceConfiguration.markExhausted and trace the call from ComputeEngineComputerLauncher.launch, focusing on the ClientUtil.nameFromSelfLink usage shown in the stack trace. Add or run the regression test for an instance-template configuration with an empty machine type and verify that exhaustion marking, fallback-zone provisioning, and node cleanup proceed without the exception.

Written by the indexing model from the issue text.