base-files: fix merge of passwd/shadow/group lines with trailing colons

base-files: fix merge of passwd/shadow/group lines with trailing colons

Empty trailing fields get lost when the lines are split and merged again
at colons, resulting in unparsable entries. Only use the split fields for
matching against the other file, but emit the original line unchanged
to fix the issue.

Fixes: de7ca7dafadf ("base-files: merge /etc/passwd et al at sysupgrade config restore")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
(cherry picked from commit 9bbaa6f2c0419739fb85d48d0f527cb1622946ee)