[SPARK-4828] [SQL] sum and avg on empty table should always return null

[adrian-wang]

So the optimizations are not valid. Also I think the optimization here is rarely encounter, so removing them will not have influence on performance.

Can we merge #3445 before I add a comparison test case from this?

[SparkQA]

Test build #24357 has started for PR 3675 at commit 42df763.

• This patch merges cleanly.

[SparkQA]

Test build #24357 has finished for PR 3675 at commit 42df763.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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[liancheng]

LGTM, thanks!

[marmbrus]

Thanks! Merged to master.