cp: treat emptystring-file as a regular missing file, not as invocation error

[BenWiederhake]

Discovered during #6176. Example:

$ rm -rf target/test/; mkdir target/test/; cp LICENSE "" README.md target/test/; echo "EXITCODE GNU: $?"; ls -l target/test/; echo "(end)"      
cp: cannot stat '': No such file or directory
EXITCODE GNU: 1
total 12
-rw-r--r-- 1 user user 1056 Apr  1 21:12 LICENSE
-rw-r--r-- 1 user user 7973 Apr  1 21:12 README.md
(end)
$ rm -rf target/test/; mkdir target/test/; cargo run cp LICENSE "" README.md target/test/; echo "EXITCODE uutils: $?"; ls -l target/test/; echo "(end)"
error: a value is required for '[paths]...' but none was supplied

For more information, try '--help'.
EXITCODE uutils: 1
total 0
(end)

In the above example, GNU cp tried its best to copy the two reasonably-named files, and reported the emptystring-named file as a regular error. In contrast, uutils cp refused to even start copying.

I feel like both behaviors are reasonable; but if we want uutils to bee a drop-in replacement, then it should also try its best to copy all files.