Fix empty array conversion in GenericFunc

[nobu-k]

The current version had a problem that passing an empty array to a UDF resulted in receiving nil instead of the empty array.

func F(a data.Array) data.Array {
  return a
}

udf.RegisterGlobalUDF("f", udf.MustConvertGeneric(F))

>>> EVAL f([]);
null

I fixed the root cause and other issues related to nil handling.

[disktnk]

LGTM!