[SPARK-3362][SQL] Fix resolution for casewhen with nulls.

Current implementation will ignore else val type.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #2245 from adrian-wang/casewhenbug and squashes the following commits:

3332f6e [Daoyuan Wang] remove wrong comment
83b536c [Daoyuan Wang] a comment to trigger retest
d7315b3 [Daoyuan Wang] code improve
eed35fc [Daoyuan Wang] bug in casewhen resolve