[fix] Layer column config: sometimes a suggested field pair will hard crash

[igorDykhta]

• Fixes a hard crash that can occur when the user selects a suggested field pair for a layer geometry column that would lead to an undefined column in the base layer assignColumnPairs method.

• The solution adds an additional short-circuit in the base layer method assignColumnPairs if that is the case, but more importantly also first prevents the UI from providing invalid suggested field pair choices that would lead to this condition.

• Adds a couple of test cases to help document to devs what the underlying findPointFieldPairs data method is intended to do.