Fuzz filter implementation respects data validity

[robert3005]

Validity::from_iter will never produce non nullable.

assertion `left == right` failed: {} != {} at index 0, lhs is vortex.struct(0x04) rhs is vortex.struct(0x04) in step 0
  left: Scalar { dtype: Struct(StructDType { names: [], dtypes: [] }, Nullable), value: ScalarValue(List([])) }
 right: Scalar { dtype: Struct(StructDType { names: [], dtypes: [] }, NonNullable), value: ScalarValue(List([])) }