Skip data parsing when squeue times out

[jordap]

The collector crashes when squeue times out since the runShellCommand with the squeue query returns None.

This PR ensures squeue isn't timing out before parsing the results by checking data is not None.

Not tested in production.

[jordap]

Btw, another option here is to increase the squeue timeout, which is always 1 second.

Not mentioning that in the warning message because 1) there is no configuration option to change it and requires changing the code, 2) longer squeue timeouts increase the chances of mismatched readings between the first and second squeue, and 3) timeout needs to be consistent with the scrape interval so that scrape_interval >= squeue_timeout * 2 (since we are executing squeue twice).

The last point also makes me think we should recommend the minimum scrape interval with squeue mode to be at least 2 seconds.