[SPARK-9372] [SQL] Filter nulls in join keys

This PR adds an optimization rule, `FilterNullsInJoinKey`, to add `Filter` before join operators to filter out rows having null values for join keys.

This optimization is guarded by a new SQL conf, `spark.sql.advancedOptimization`.

The code in this PR was authored by yhuai; I'm opening this PR to factor out this change from #7685, a larger pull request which contains two other optimizations.

Author: Yin Huai <yhuai@databricks.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #7768 from JoshRosen/filter-nulls-in-join-key and squashes the following commits:

c02fc3f [Yin Huai] Address Josh's comments.
0a8e096 [Yin Huai] Update comments.
ea7d5a6 [Yin Huai] Make sure we do not keep adding filters.
be88760 [Yin Huai] Make it clear that FilterNullsInJoinKeySuite.scala is used to test FilterNullsInJoinKey.
8bb39ad [Yin Huai] Fix non-deterministic tests.
303236b [Josh Rosen] Revert changes that are unrelated to null join key filtering
40eeece [Josh Rosen] Merge remote-tracking branch 'origin/master' into filter-nulls-in-join-key
c57a954 [Yin Huai] Bug fix.
d3d2e64 [Yin Huai] First round of cleanup.
f9516b0 [Yin Huai] Style
c6667e7 [Yin Huai] Add PartitioningCollection.
e616d3b [Yin Huai] wip
7c2d2d8 [Yin Huai] Bug fix and refactoring.
69bb072 [Yin Huai] Introduce NullSafeHashPartitioning and NullUnsafePartitioning.
d5b84c3 [Yin Huai] Do not add unnessary filters.
2201129 [Yin Huai] Filter out rows that will not be joined in equal joins early.

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