Handle case without observed equations

Author: hersle

src/systems/nonlinear/nonlinearsystem.jl

@@ -196,7 +196,7 @@ function calculate_jacobian(sys::NonlinearSystem; sparse = false, simplify = fal
     # observed equations may depend on unknowns, so substitute them in first
     # TODO: must do the same fix in e.g. calculate_hessian?
     # TODO: rather keep observed derivatives unexpanded, like "Differential(obs)(expr)"?
-    obs = map(eq -> eq.lhs => eq.rhs, observed(sys)) |> todict
+    obs = Dict(eq.lhs => eq.rhs for eq in observed(sys))
     rhs = map(eq -> fixpoint_sub(eq.rhs, obs), equations(sys))
     vals = [dv for dv in unknowns(sys)]