FakeBkgTools: fix initialization failure for one-letter systematics/parameters
As reported by @kiabelin (thanks!), this doesn't work as it should:
<syst affects="electron,muon"> DiBoson,W,stop,met,d0sig,ttbar,Z</syst>
due to a bug in the splitting+trimming of the comma-separated values. This affects only parameters with a one-character name, and leads to an initialization failure, so no impact on physics.