Adds NULL typing to JOIN operators

[rchowell]

Relevant Issues

#1238

Description

When performing a LEFT (or RIGHT join), we must type the right-hand-side (or left-hand-side) as nullable because these joins pad the resultant tuple with NULLs if the condition fails to match.

This logic will type a non-struct value as nullable, but will type the fields of a struct value as nullable. This allows for SQL compatibility of the JOIN operators. Details in Issue #1238

Other Information

• Updated Unreleased Section in CHANGELOG: [YES/NO]
  No

• Any backward-incompatible changes? [YES/NO]
  This changes the behavior of PlanTyperV0

• Any new external dependencies? [YES/NO]
  No

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES/NO]
  Yes

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (d37da34)	8432f04	+/-
% Passing	92.33%	92.33%	0.00%
✅ Passing	5372	5372	0
❌ Failing	446	446	0
🔶 Ignored	0	0	0
Total Tests	5818	5818	0

Number passing in both: 5372

Number failing in both: 446

Number passing in Base (d37da34) but now fail: 0

Number failing in Base (d37da34) but now pass: 0