ext/ldap: Allow default host from ldap.conf to work.

ext/ldap: Allow default host from ldap.conf to work.

This fixes an regression introduced in
e7af0fe1eb89e40671e86a588aa1b78607b85461. Previously, calling
ldap_connect() with no parameters would pass NULL to ldap_init(),
which causes it to use the default host specified in
/etc/ldap/ldap.conf (on Ubuntu).

When the code changed to use ldap_initialize(), it initialized a uri,
even if there were no parameters passed to ldap_connect(). Because of
this, there's no way to pass a NULL into ldap_initialize(), making it
impossible to use the default uri from ldap.conf.

This commit bypasses the uri creation when there is no host argument,
passing on a NULL to ldap_initialize() which restores the old PHP 5.5
behavior.