Prevent LDAP authorization crash when parsing null valued attributes

RFC4511 allows LDAP searches to return empty attributes which have no values:

4.1.7. Attribute and PartialAttribute

Attributes and partial attributes consist of an attribute description
and attribute values. A PartialAttribute allows zero values, while
Attribute requires at least one value.

PartialAttribute ::= SEQUENCE {
type AttributeDescription,
vals SET OF value AttributeValue }

4.5.2. Search Result

The results of the Search operation are returned as zero or more
SearchResultEntry and/or SearchResultReference messages, followed by
a single SearchResultDone message.

SearchResultEntry ::= [APPLICATION 4] SEQUENCE {
objectName LDAPDN,
attributes PartialAttributeList }

PartialAttributeList ::= SEQUENCE OF
partialAttribute PartialAttribute

SearchResultReference ::= [APPLICATION 19] SEQUENCE
SIZE (1..MAX) OF uri URI

SearchResultDone ::= [APPLICATION 5] LDAPResult

Servers conforming to this specification do not need to return any values for a particular attribute.

libldap appears to return a NULL pointer when ldap_get_values_len is called for an attribute without values, instead of an array containing a NULL pointer. libldap more commonly returns a NULL pointer on error. However, this condition is not considered an error, so no result code is set on the LDAP session handle.

We should correctly handle this case.