Region Union with Non-existent items

Hello everyone,

I have a question about the "Region Union" component, as I am trying to combine a few parametrically defined areas. It is in the hope of defining a walking distance with obstacles in mind.

These areas are sometimes null according to the definition (circles of radii=0) and result as "invalid". I am trying to union them with other areas which always exist, but the operation result is blank, not an invalid shape, not a null object, just blank. Ultimately I'd like the "Region Union" component to result in either the combination of shapes, or in at least the existing shape.

I wanted to use the "Replace Nulls" component after the "Region Union" component to sort this issue out, however because the result of the union is blank (not a null object), it doesn't work. 

I have attached the gh file (ironically named "No Bugs") but the expression is a bit lengthy unfortunately, I have grouped the problematic RUnion component. The error occurs if you toggle the size of the initial rectangle.

Thank you very much in advance for any help at all.