APIv4 - Automatically coalesce potentially null field values in equations

[colemanw]

Overview

Follow-up to #21551
Fixes APIv4 SQL Equations to treat null values as 0.

Before

Any null value would return null for the entire equation.

After

Null values treated as 0.

[civibot]

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[colemanw]

@eileenmcnaughton this is a fairly simple solution to what we talked about.

[eileenmcnaughton]

@colemanw these 2 test fails relate

api.v4.Action.ContactApiKeyTest.testGetApiKey
api.v4.Action.SqlExpressionTest.testSelectEquations

[eileenmcnaughton]

test this please

[eileenmcnaughton]

As use cases go this is a bit weak - but it worked