[REF] [merge custom fields] Only call function when applicable (well tested code)

[eileenmcnaughton]

Overview

[REF] Only call function when applicable

Before

The function does an early return when called for non-custom fields

After

The function is not called for non-custom fields

Technical Details

Big whitespace change, small code change

Comments

[civibot]

(Standard links)

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[demeritcowboy]

merge conflict

[eileenmcnaughton]

@demeritcowboy rebased