Coercing an undefined value to a numeric value causes problems with rendering

When an undefined value is somehow coerced to be set on a field expecting something numeric (stuff I've tested being x/y at least, but I imagine this also affects scale and other things), then Bad Things can happen.

See attached testcase, and notice that the green rectangle vanishes unless the 'x' property is not set.

Note that in a debug build, you'll see some of these:
[W] default: QTransform::translate:414 - QTransform::translate with NaN called

I'm not sure what the optimal behavior is here, but I don't think this is it.