Impossible to invoke a method ("getUrl") on a null variable
You can do it like this:

.bg{
    background-image: {{ entry.siteBg.first().getUrl('bodyBg')|default(siteBgFallback.siteBg.first().getUrl('bodyBg)}}
}


And the same for the other one. What you are doing is the following:


Set an item that can be empty (hence the |default). 
If the item is empty go to the fallback (the part between the default brackets).


For more info on the default filter check here