Add 'optionToArray' helper

[ghost]

Adds a helper function for converting T | undefined to T[].

[rbuckton]

Is this really necessary? It will be inlined to the same code and its not much shorter than writing x ? [x] : [].

[ghost]

Perhaps not, though in the linked PR I want to write optionToArray(getActionsForAddMissingInitializer(context, propertyDeclaration, newLineCharacter)) instead of applying to a local variable -- would be nice to be able to write let x = getActions... in x ? [x] : [], but we could do it in reverse with a lambda (x => x ? [x] : [])(...) (in other words, write the definition of optionToArray inline).