allow null values in `list` parameter of _.object

Previously _.object([['a', 'b'], null]) would crash. Now it will
ignore the null value and just give {'a': 'b'}. This is especially
convenient if you are chaining it with _.map and you want to
remove some values from an object:

> _.object(_.map({'a': 100, 'b': 150}, function(v, k) {
  return v > 120 ? [k, v] : null
  }))
  { b: 150 }