[SPARK-15370][SQL] Update RewriteCorrelatedScalarSubquery rule to fix COUNT bug

## What changes were proposed in this pull request?
This pull request fixes the COUNT bug in the `RewriteCorrelatedScalarSubquery` rule.

After this change, the rule tests the expression at the root of the correlated subquery to determine whether the expression returns NULL on empty input. If the expression does not return NULL, the rule generates additional logic in the Project operator above the rewritten subquery.  This additional logic intercepts NULL values coming from the outer join and replaces them with the value that the subquery's expression would return on empty input.

## How was this patch tested?
Added regression tests to cover all branches of the updated rule (see changes to `SubquerySuite.scala`).
Ran all existing automated regression tests after merging with latest trunk.

Author: frreiss <frreiss@us.ibm.com>

Closes #13155 from frreiss/master.