fix: fix creation of JumpProblems with no parameters

Author: AayushSabharwal

src/systems/jumps/jumpsystem.jl

@@ -508,7 +508,7 @@ end
 function JumpSysMajParamMapper(js::JumpSystem, p; jseqs = nothing, rateconsttype = Float64)
     eqs = (jseqs === nothing) ? equations(js) : jseqs
     paramexprs = [maj.scaled_rates for maj in eqs.x[1]]
-    psyms = reduce(vcat, reorder_parameters(js, parameters(js)))
+    psyms = reduce(vcat, reorder_parameters(js, parameters(js)); init = [])
     paramdict = Dict(value(k) => value(v) for (k, v) in zip(psyms, vcat(p...)))
     JumpSysMajParamMapper{typeof(paramexprs), typeof(psyms), rateconsttype}(paramexprs,
         psyms,

test/jumpsystem.jl

@@ -231,3 +231,32 @@ let
     jtoj = eqeq_dependencies(jdeps, vdeps).fadjlist
     @test jtoj == [[1, 2, 4], [1, 2, 4], [1, 2, 3, 4], [1, 2, 3, 4]]
 end
+
+# Create JumpProblems for systems without parameters
+# Issue#2559
+@parameters k
+@variables X(t)
+rate = k
+affect = [X ~ X - 1]
+
+crj = ConstantRateJump(1.0, [X ~ X - 1])
+js1 = complete(JumpSystem([crj], t, [X], [k]; name = :js1))
+js2 = complete(JumpSystem([crj], t, [X], []; name = :js2))
+
+maj = MassActionJump(1.0, [X => 1], [X => -1])
+js3 = complete(JumpSystem([maj], t, [X], [k]; name = :js2))
+js4 = complete(JumpSystem([maj], t, [X], []; name = :js3))
+
+u0 = [X => 10]
+tspan = (0.0, 1.0)
+ps = [k => 1.0]
+
+dp1 = DiscreteProblem(js1, u0, tspan, ps)
+dp2 = DiscreteProblem(js2, u0, tspan)
+dp3 = DiscreteProblem(js3, u0, tspan, ps)
+dp4 = DiscreteProblem(js4, u0, tspan)
+
+jp1 = JumpProblem(js1, dp1, Direct())
+jp2 = JumpProblem(js2, dp2, Direct())
+jp3 = JumpProblem(js3, dp3, Direct())
+jp4 = JumpProblem(js4, dp4, Direct())