`JobInstanceDao#getJobInstanceCount` throws `NoSuchJobException` instead of returning zero

[kzander91]

This is a breaking behaviour change without any benefit imho. While this behaviour has been documented in the method's Javadoc, I don't see the reason behind it: Counting the number of invocations for a job that has never run should return 0, not throw an exception.

This change makes the JobInstanceDao/JobRepository classes more cumbersome to use.
With Batch 5:

int count = jobExplorer.getJobInstanceCount("myJob");
// do stuff with count, `0` is a possibility

// or in tests
assertThat(jobExplorer.getJobInstanceCount("myJob")).isZero();
// run job, then assert again
assertThat(jobExplorer.getJobInstanceCount("myJob")).isOne();

With Batch 6:

int count;
try {
  count = jobExplorer.getJobInstanceCount("myJob");
} catch (NoSuchJobException _) {
  count = 0;
}

// or in tests
assertThatExceptionOfType(NoSuchJobException.class)
  .isThrownBy(() -> jobExplorer.getJobInstanceCount("myJob"));
// run job, then assert again
assertThat(jobExplorer.getJobInstanceCount("myJob")).isOne();

Please consider reverting this change.