BF: fix argument parsing

Paul McCarthyrequested to merge
paulmc-master-patch-82198 into master

Reported here: https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?A2=FSL;c858e06b.2112 and here: https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?A2=FSL;495523a1.2210

I forgot to apply the fix after the first report.

Occasionally bedpostx_postproc_gpu.sh will be passed an empty string as an argument, which causes the -z "$2" test to return false. The test needs to check whether the $2 variable is set, as opposed to testing whether it is non-empty.

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