[SPARK-14191][SQL] Remove invalid Expand operator constraints

`Expand` operator now uses its child plan's constraints as its valid constraints (i.e., the base of constraints). This is not correct because `Expand` will set its group by attributes to null values. So the nullability of these attributes should be true.

E.g., for an `Expand` operator like:

    val input = LocalRelation('a.int, 'b.int, 'c.int).where('c.attr > 10 && 'a.attr < 5 && 'b.attr > 2)
    Expand(
      Seq(
        Seq('c, Literal.create(null, StringType), 1),
        Seq('c, 'a, 2)),
      Seq('c, 'a, 'gid.int),
      Project(Seq('a, 'c), input))

The `Project` operator has the constraints `IsNotNull('a)`, `IsNotNull('b)` and `IsNotNull('c)`. But the `Expand` should not have `IsNotNull('a)` in its constraints.

This PR is the first step for this issue and remove invalid constraints of `Expand` operator.

A test is added to `ConstraintPropagationSuite`.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #11995 from viirya/fix-expand-constraints.

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