nil binding should be treated as sql null

mruby's nil is MRB_TT_FALSE, not MRB_TT_UNDEF, so binding nil is treated as false, then it'll be bind as 0.
"/* {MRB_TT_UNDEF, "undef"}, */ /* internal use: #undef; should not happen */" https://github.com/mruby/mruby/blob/444e4b7976d629663d1020de65280e01853a2ca4/src/object.c