Handle null for fieldContext within resolvePotentialPartialArray

[faustbrian]

When resolvePropertyValue calls resolvePotentialPartialArray there is an exception thrown if $fieldContext is null. This is possible because the parameters of resolvePropertyValue contain ?TransformationContext $fieldContext which means it can be null but wasn't allowed to be null within resolvePotentialPartialArray.

This fixes that and was tested by making our own test suite pass again and errors going away when using our application.

[rubenvanassche]

Thanks!