`symbolic_linear_solve` returns Inf if var is missing

[baggepinnen]

solving for a variable that does not appear in the equation gives a nonsensical result

julia> @variables a b
2-element Vector{Num}:
 a
 b

julia> symbolic_linear_solve(0 ~ a, b)
(-1//0)*a

[ChrisRackauckas]

0 ~ a + 0b -0b ~ 1*a b ~ -1/0 * a

[ChrisRackauckas]

We can make this return NaN