string-list: split-then-remove-empty can be done while splitting

string-list: split-then-remove-empty can be done while splitting

Thanks to the new STRING_LIST_SPLIT_NONEMPTY flag, a common pattern
to split a string into a string list and then remove empty items in
the resulting list is no longer needed.  Instead, just tell the
string_list_split*() to omit empty ones while splitting.

Signed-off-by: Junio C Hamano <gitster@pobox.com>