[config] Tune "OptionStringList::fromString()"
[config] Tune "OptionStringList::fromString()"

Sample two line input:
", a, b, c , d, e, , f,
g, h"

Before: Output list contains 4 empty elements:
", a, b, c, , d, e, , f, , g, h"

After: Output list contains 2 empty elements:
", a, b, c, d, e, , f, g, h"

Closes: #755
Approved by: m-blaha

rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libhif