Add NULL key handling in custom compare function wrapper
Add NULL key handling in custom compare function wrapper

- The cmp function wrapper needs to filter out NULL keys before it
calls the user-defined compare function.

sushmaad/forestdb,uvenum/forestdb,nvoron23/forestdb,yyangpan/forestdb,seem-sky/forestdb,joliss/forestdb,joliss/forestdb,wurikiji/forestdb,nvoron23/forestdb,zhaog/forestdb,couchbase/forestdb,hisundar/forestdb,seem-sky/forestdb,wurikiji/forestdb,hisundar/forestdb,jinxuan/forestdb,zhaog/forestdb,uvenum/forestdb,yyangpan/forestdb,jinxuan/forestdb,sushmaad/forestdb,couchbase/forestdb