BigQuery: create() throws NullPointerException on a duplicate job id outside the US multi-region (java-bigquery#3034 not fixed by #3035)

#### Environment details

1. API: BigQuery (`java-bigquery`)
2. OS type and version: macOS 15 / Linux (not OS-specific)
3. Java version: 17
4. Version: google-cloud-bigquery 2.68.0, and present on current `main`
(`java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/BigQueryImpl.java`
lines 609–611)

#### Summary

`BigQuery.create(JobInfo)` with an explicit job id throws
`NullPointerException: Cannot invoke "com.google.cloud.bigquery.Job.getStatistics()" because "job" is null`
when the id already exists and the job lives outside the US multi-region while the `JobId` carries
no location — masking the real `Already Exists: Job project:location.id` error.

This is the failure googleapis/java-bigquery#3034 reported. That issue was closed by
googleapis/java-bigquery#3035, which does not cover this path:

- #3035's production change narrows the re-fetch from `getJob(jobId)` to
`getJob(jobId, JobOption.fields(JobField.STATISTICS))`. Narrowing the requested fields cannot
turn a null return into a job, and #3034's stack trace shows the receiver `job` itself was null.
- #3035's added test (`testCreateJobTryGetNotRandom`) mocks the RPC `getJob` **returning a job**,
so only the found-job path is covered; the null return has no test.

The NPE therefore still reproduces, deterministically, on 2.68.0 — which contains #3035's change.

#### Root cause

`jobs.get` resolves a `JobId` that names no location against the US multi-region only. When the
duplicated job's location was inferred from the statement (a query or load touching a regional
dataset), the duplicate-id handler's re-fetch misses it, `getJob` returns null, and
`job.getStatistics()` throws. The `idRandom` branch a few lines below already handles exactly this
case with `if (job == null) { throw createException; }`; the fixed-id branch lacks the same guard.

#### Steps to reproduce

Run the code below against any project with a dataset outside the US multi-region (reproduced
against a `us-central1` dataset). No location is set anywhere; the region comes from the query.

```java
BigQuery bq = BigQueryOptions.newBuilder().setProjectId(project).build().getService();
String sql = "SELECT COUNT(*) FROM `" + project + "." + regionalDataset + ".INFORMATION_SCHEMA.TABLES`";
String id = "npe_repro_" + System.currentTimeMillis();

bq.create(JobInfo.of(JobId.newBuilder().setProject(project).setJob(id).build(),
QueryJobConfiguration.of(sql))); // ok — server infers us-central1
bq.create(JobInfo.of(JobId.newBuilder().setProject(project).setJob(id).build(),
QueryJobConfiguration.of(sql))); // NullPointerException
```

Control arm: setting the location on the second create's `JobId` makes the handler find the job
and return it — the intended duplicate-id behaviour — which isolates the defect to the
location-less re-fetch.

#### Stack trace

```
java.lang.NullPointerException: Cannot invoke "com.google.cloud.bigquery.Job.getStatistics()" because "job" is null
at com.google.cloud.bigquery.BigQueryImpl.create(BigQueryImpl.java:611)
at com.google.cloud.bigquery.BigQueryImpl.create(BigQueryImpl.java:511)
```

#### Expected behaviour and fix

When the re-fetch returns null, fall through to the original exception, as the `idRandom` branch
does. Verified against BigQuery with the guard patched in: the NPE becomes
`BigQueryException: Already Exists: Job flink-gcp:us-central1.npe_repro_a_…`, whose message names
the job's actual location. I can send the PR.

Start in java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/BigQueryImpl.java around lines 609–611, then compare the fixed-id branch with the nearby idRandom null handling. Review testCreateJobTryGetNotRandom and add coverage for a null re-fetch; done means a regional duplicate produces the original Already Exists error instead of a NullPointerException.

索引モデルが issue の本文から書いたものです。