Fix ensureFullBlockClosureClass for older images

[ErikOnBike]

My tiny CodeParadise image fails because ensureFullBlockClosureClass() will put undefined in the FullBlockClosure entry of the special objects array. This additional check prevents this.

[ErikOnBike]

Hi Fabio. Well actually they don't work for me. But that's because I'm using an old image without FBC :-). But because it is missing, an 'undefined' value was written to the special objects array. Other code would 'break' on this. So the additional check prevents the replacement of the value in the special objects array. The current value in splObs is 'nil' in my old image (of course). But 'nil' is not undefined :-D. Thx for approval!

[fniephaus]

Ah right! Looks like I drove by too fast. 😉 The additional check still makes sense!