fix: allow nullable uid in InetOrgPerson

[ezhilnn]

Fixes #19370

Problem

uid is marked @Nullable on the field but two places contradicted
this:

• getUid() called Objects.requireNonNull(), throwing NPE if uid is null
• Essence(DirContextOperations) constructor called Assert.notNull(),
  throwing IllegalArgumentException if uid is absent from LDAP context

According to the LDAP inetOrgPerson schema, uid is not a mandatory
field. This behavior also regressed from pre-7.1.0 behavior (introduced
in commit c5632cc).

Changes

• getUid() now returns @Nullable String and returns the field directly
• Constructor no longer asserts uid is non-null; skips setUid() if absent
• setUid() now accepts @Nullable String and guards username mapping
  with a null check

Testing

• Added testNullUidIsAllowed() to verify null uid no longer throws