Passing function to `derivative` results in zero

[Aaron Kaw]

Not sure if this is a bug, but I think this should either throw an error or be fixed.

Setup:

using Symbolics: derivative
f(x) = x^2
@variables x

and then

derivative(f, x)

gives 0,

derivative(f(x), x)

gives Num(2x).

The former should error or be fixed, right?

[Christopher Rackauckas]

Yes, we should make it error on non-Num or BasicSymbolic