entry: refactor how we remove items for delayed checkouts

entry: refactor how we remove items for delayed checkouts

When finalizing a delayed checkout, we sort out several strings from the
passed-in string list by first assigning the empty string to those
filters and then calling `string_list_remove_empty_items()`. Assigning
the empty string will cause compiler warnings though as the string is
a `char *` once we enable `-Wwrite-strings`.

Refactor the code to use a `NULL` pointer with `filter_string_list()`
instead to avoid this warning.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>