parsing nil needs to not coredump

[lamont-granquist]

this coredumps:

FFI_Yajl::Parser.parse(nil)

other gems throw exceptions:

> JSON.parse(nil)

TypeError: no implicit conversion of nil into String

from /Users/lamont/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/json/common.rb:155:in `initialize'
> Yajl::Parser.parse(nil)

Yajl::ParseError: input must be a string or IO

from /Users/lamont/.rvm/gems/ruby-2.1.2/gems/yajl-ruby-1.2.1/lib/yajl.rb:37:in `parse'