[REF] Clarify variable & tighten use.

[eileenmcnaughton]

Overview

Minor code tidy up in heavily tested dedupe code

Before

$customTableToCopyFrom is always an array - but the code handles the possibility it is null

After

$customTableToCopyFrom is strictly an array and possibility of null handling is removed

Technical Details

moveContactBelongings is only called from one place which passes in all the function variables, so we don't need defaults.

The last parameter, is retrieved from self::getAffectedCustomTables which always returns an
array - so all the handling for it being NULL can be removed....

Comments

[civibot]

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[colemanw]

Code looks good and tests should cover it.