apps: allow empty attribute values with -subj

Historically (i.e., OpenSSL 1.0.x), the openssl applications would
allow for empty subject attributes to be passed via the -subj argument,
e.g., `opensl req -subj '/CN=joe/O=/OU=local' ...`.  Commit
db4c08f0

 applied a badly needed rewrite
to the parse_name() helper function that parses these strings, but
in the process dropped a check that would skip attributes with no
associated value.  As a result, such strings are now treated as
hard errors and the operation fails.

Restore the check to skip empty attribute values and restore
the historical behavior.

Document the behavior for empty subject attribute values in the
corresponding applications' manual pages.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7349)