Evaluate dif for inequalities only if arguments are reals

fixes #8288
I would rather have the Ellipse logic changed (e.g. a < b in code changed to a - b < 0) rather than impose the extra burden of requiring "real" to be specified for variables. DiracDelta, as I recall, does this in assuming its variables are real if they aren't specified. This is a safe assumption all throughout the geometry module, too.