[SPARK-18070][SQL] binary operator should not consider nullability when comparing input types

## What changes were proposed in this pull request?

Binary operator requires its inputs to be of same type, but it should not consider nullability, e.g. `EqualTo` should be able to compare an element-nullable array and an element-non-nullable array.

## How was this patch tested?

a regression test in `DataFrameSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15606 from cloud-fan/type-bug.