Allow incomplete initialization

Author: YingboMa

src/variables.jl

@@ -12,9 +12,12 @@ applicable.
 """
 function varmap_to_vars(varmap, varlist; defaults=Dict(), check=true)
     # Edge cases where one of the arguments is effectively empty.
-    if varmap isa DiffEqBase.NullParameters || varmap === nothing || isempty(varmap)
+    is_incomplete_initialization = varmap isa DiffEqBase.NullParameters || varmap === nothing
+    if is_incomplete_initialization || isempty(varmap)
         if isempty(defaults)
-            check && (isempty(varlist) || throw_missingvars(varlist))
+            if !is_incomplete_initialization && check
+                isempty(varlist) || throw_missingvars(varlist)
+            end
             return nothing
         else
             varmap = Dict()

test/sdesystem.jl

@@ -29,7 +29,7 @@ solexpr = solve(eval(probexpr),SRIW1(),seed=1)
 @test all(x->x==0,Array(sol-solexpr))
 
 # Test no error
-@test_nowarn SDEProblem(de,zeros(3),(0, 10.0),zeros(3))
+@test_nowarn SDEProblem(de,nothing,(0, 10.0))
 
 noiseeqs_nd = [0.01*x 0.01*x*y 0.02*x*z
                σ      0.01*y   0.02*x*z