Correctly handle uninitialized value in stats generation.

[svenoe]

Setting up a ticket accumulation statistic without configuring the y axis, results in uninitialized error warnings:

Use of uninitialized value $ColumnName in string ne at /opt/otobo/Kernel/System/Stats.pm line 3621.
Use of uninitialized value $ColumnNameOld in string ne at /opt/otobo/Kernel/System/Stats.pm line 3621.

otobo/Kernel/System/Stats.pm

Line 3619 in 4fa8073

next COLUMNNAMEOLD if $ColumnNameOld ne $ColumnName;

when executing e.g.:

bin/otobo.Console.pl Maint::Stats::Generate --number 10010 --target-directory tmpstat --language de

Please compare the $HeadArrayRef to a stat with set y axis, and deal with this appropriately (I'm not certain whether this is even valid out of my head, it might be, though).