fix: correctly calculate join output schema nullability

[alamb]

Which issue does this PR close?

Part of #2778

Rationale for this change

apache/arrow-rs#1888 in arrow added validation to RecordBatch if the schema's declared nullability is different than its actual nullability.

This caught that the output schema calculation for joins is incorrect -- specifically, LEFT/RIGHT/FULL joins can introduce nulls even if the input schema is not nullable.

For example, given the following non-null input:

a
1

b
2

This query:

SELECT * FROM a LEFT JOIN b

Produces a null on b (though a is non nullable if a is non nullable in the input) and thus b must be marked nullable

a	b
1	NULL

What changes are included in this PR?

1. Account for NULLs introduced in joins in output schema calculation
2. Tests

Are there any user-facing changes?

I don't think so (except more correct null schema marking)

[codecov-commenter]

Codecov Report

Merging #2803 (89fa13a) into master (533e2b4) will increase coverage by 0.09%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #2803      +/-   ##
==========================================
+ Coverage   85.11%   85.20%   +0.09%     
==========================================
  Files         273      274       +1     
  Lines       48240    48590     +350     
==========================================
+ Hits        41060    41402     +342     
- Misses       7180     7188       +8     

Impacted Files	Coverage Δ
datafusion/core/src/physical_plan/join_utils.rs	93.61% <88.46%> (-3.20%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	74.00% <0.00%> (-0.40%)	⬇️
datafusion/optimizer/src/reduce_outer_join.rs	99.39% <0.00%> (ø)
datafusion/core/src/execution/context.rs	78.56% <0.00%> (+0.02%)	⬆️
datafusion/core/tests/sql/joins.rs	99.31% <0.00%> (+0.20%)	⬆️
datafusion/core/src/physical_plan/metrics/value.rs	87.43% <0.00%> (+0.50%)	⬆️

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[xudong963]

Thank you @alamb