Add throw_if_scalar_and_constant_not_zero

[blegat]

Extracted from #1287

[odow]

Needs tests for SingleVariable and AbstractVectorFunction.

Can't they also be simplified to:

throw_if_scalar_and_constant_not_zero(::Any, ::Any) = nothing

[blegat]

It could but then an invalid call would return nothing instead of a MethodError

[odow]

How could it get a method error? And when would a method error be preferable?

[blegat]

If you call it with a scalar function and a vector set. Or with any type like (1, Int)

[odow]

That should be caught at the original caller? We shouldn't rely on this method to throw a method error. It would violate the MethodError principle.

[blegat]

Since this is a documented function, the user might call this directly in which case it would get the MethodError at the first method called so it agrees with the principle.
I don't think the places where it is called from the MOI source code will ever never hit the MethodError.