APIv4 - Correctly return null values from DAO save actions

[colemanw]

Overview

Make sure the value "null" is not returned as a string
Don't return null values from saved DAO objects unless it is in params or reload is set to true

Before

Some weirdness was coming back from APIv4 Create/Update/Save actions. null values were sometimes the string "null" (which is a workaround at the DB layer we don't want bleeding into the api).
It would also return some misleading data during update operations, returning null for fields which may not be null in the db but were not set in the update.

After

Ensures null is not returned as a string.
Ensures unknown values are not returned at all from update ops instead of being returned as null.

Technical Details

Test added.

[civibot]

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[seamuslee001]

Test fail is unrelated. Changes look fine to me

[artfulrobot]

• General standards
  • Explain PASS
  • User impact (r-user) PASS: This will be better for future users. Users who have put in place workarounds for the quirks before this PR won't be affected; they'll get the correct NULL already.
  • Documentation PASS: The changes do not require documentation because the result of the PR is that it works as documented :-)
  • Run it (r-run) UNREVIEWED
• Developer standards
  • Technical impact (r-tech)
    PASS: The change preserves compatibility with existing callers/code/downstream.
  • Code quality (r-code)
    • PASS
  • Maintainability (r-maint)
    • PASS: Great to have the test in place.
  • Test results (r-test)
    • PASS: The test results have failures, but these have been individually inspected and found to be irrelevant.