Ticket #47772 empty modify returns LDAP_INVALID_DN_SYNTAX
    
    https://fedorahosted.org/389/ticket/47772
    Reviewed by: mreynolds (Thanks!)
    Branch: 389-ds-base-1.2.11
    Fix Description: Do not call normalize_mods2bvals if mods is NULL - just allow
    the empty modify op to proceed.  Since normalize_mods2bvals only cares about
    DN syntax attributes, it is appropriate to return LDAP_INVALID_DN_SYNTAX if
    normalize_mods2bvals returns NULL.
    Since this fix allows NULL mods to proceed throughout the code, I checked for
    all places that SLAPI_MODIFY_MODS was used, to make sure we don't try to
    dereference NULL mods, and to make sure the NULL mods were handled correctly
    otherwise.
    Platforms tested: RHEL6 x86_64
    Flag Day: no
    Doc impact: no
    (cherry-picked with conflicts from f5730129dfbc4ef3808735051b12cbfda518f4fb)
    (cherry picked from commit 469c9b9c68806e46b14595da5aec49517af60028)
    (cherry picked from commit 60c36f1c6aea1913bd7f719b9efb892092317917)
    (cherry picked from commit 1a39cf36137d522f81bc003d07c21da53371a69d)