feat: allow null field values in loadManyByFieldEqualityConjunctionAsync

[wschurman]

Why

A feature request from @jkhales for an API endpoint within the Expo server.

Since the results for this type of load aren't stored in a dataloader or cached, we can just pipe null fieldValues directly through to the database adapters for load, and it is their duty to handle them for this type of load.

How

This ended up being slightly more difficult than anticipated but not too bad. Complications:

• ANY(?) cannot contain NULL so it needs to be split into separate OR WHERE but within the same conjunctive clause.
• Replicating the default NULLS FIRST/LAST postgres behavior in StubDatabaseAdapter required checking for null explicitly: https://www.postgresql.org/docs/8.3/queries-order.html#:~:text=The-,NULLS%20FIRST,-and%20NULLS%20LAST

Test Plan

Run tests.

[codecov]

Codecov Report

Merging #130 (9ef9fe7) into master (5401be0) will increase coverage by 0.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   95.31%   95.76%   +0.45%     
==========================================
  Files          70       70              
  Lines        1772     1796      +24     
  Branches      208      217       +9     
==========================================
+ Hits         1689     1720      +31     
+ Misses         81       75       -6     
+ Partials        2        1       -1     

Flag	Coverage Δ
integration	95.76% <100.00%> (+0.45%)	⬆️
unittest	95.76% <100.00%> (+0.45%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/entity/src/EntityDatabaseAdapter.ts	100.00% <ø> (ø)
...-adapter-knex/src/PostgresEntityDatabaseAdapter.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityLoader.ts	88.88% <100.00%> (ø)
...es/entity/src/utils/testing/StubDatabaseAdapter.ts	98.09% <100.00%> (+7.77%)	⬆️

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[jkhales]

Wow, quick turn around! Thank you!