Fix crash in bin transform with empty input table

[jonmmease]

When a transform pipeline that involves extent and bin was fed an empty table as input, the following error would occur.

Internal error: Cannot convert NULL to f64

The issue is that extent returns [NULL, NULL] in this case (which is valid) and bin would attempt to cast these elements to f64 values (which is not valid). Now we default to [0.0, 0.0] in this case (any valid values would work since the result will be an empty table).