Adds protection for setting nil values in bindings for float & bool elements

I'm seeing an issue with bindings where a property that is an NSNumber is set to nil:


    @interface Foo : NSObject
    @property (nonatomic, strong) NSNumber *boolNumber;
    @end
    ...
    Foo *foo = [[myFoo alloc] init];
    foo.boolNumber = nil;

Then the property is used as a binding to a QBoolElement. This causes an exception to be thrown, as setting the bool value in QBoolElement to nil is not allowed.

I've added a check for this where it will be set to NO in the case where it's nil.