common.getSeconds does not handle empty argument

[bhmevik]

With Slurm, the UserCPU output of sacct can be empty, for instance if a job got cancelled while it was pending. In such cases, common.getSeconds() will log an error and return -1.
Perhaps a better action is to silently return 0.
If that is not acceptable, then a test for this case should be added in createUsageRecord() in slurm.py, so it circumvents getSeconds() if the value is empty (or calls getSeconds() with '00:00').
If one wants to be more secure, one could do this only when Start and End times are the same (or Elapsed is '00:00:00'), and allow getSeconds() to log an error otherwise.

[bhmevik]

Actually, perhaps it is better just to filter out all job records where End == Start (or Elapsed == '00:00:00'), since all jobs that are cancelled while pending, look like this. And accounting jobs of zero run time is perhaps not so interesting.

[bhmevik]

Just checking for elapsed == 00:00:00 was not enough; there are jobs with elapsed > 0 but with missing UserCPU. I've created a pull request that skips jobs with zero elapsed, and which uses 00:00:00 when UserCPU field is missing: #26