fix: reject empty attribute type in ParseDN

[netliomax25-code]

1. ParseDN uses len(attr.Type) == 0 to decide whether an unescaped = starts the value, but setType accepts a segment that decodes to the empty string, so the type stays empty and a later = in the same pair re-binds it.
2. The empty leading segment is silently dropped: =cn=admin,dc=net and DC=example,=DC=net parse without error into the same structures as cn=admin,dc=net and DC=example,DC=net, and compare equal under DN.Equal. RFC 4514 section 3 defines attributeType = descr / numericoid, which is never empty, and the existing tests already expect the single-equals forms (=test,DC=example,DC=com, DC=example,=net) to fail.

Tracked type consumption with an explicit flag so a set-but-empty type falls through to the existing "incomplete type, value pair" errors. Valid DNs parse exactly as before, and the previously expected error messages are unchanged.