Fix CI failure

Author: YingboMa

src/systems/diffeqs/abstractodesystem.jl

@@ -291,7 +291,9 @@ function process_DEProblem(constructor, sys::AbstractODESystem,u0map,parammap;
     u0 = varmap_to_vars(u0map,dvs; defaults=defs)
     p = varmap_to_vars(parammap,ps; defaults=defs)
 
-    length(dvs) == length(u0) || throw(ArgumentError("States ($(length(dvs))) and initial conditions ($(length(u0))) are of different lengths."))
+    if u0 !== nothing
+        length(dvs) == length(u0) || throw(ArgumentError("States ($(length(dvs))) and initial conditions ($(length(u0))) are of different lengths."))
+    end
 
     f = constructor(sys,dvs,ps,u0;tgrad=tgrad,jac=jac,checkbounds=checkbounds,
                     linenumbers=linenumbers,parallel=parallel,simplify=simplify,

src/systems/nonlinear/nonlinearsystem.jl

@@ -211,7 +211,9 @@ function process_NonlinearProblem(constructor, sys::NonlinearSystem,u0map,paramm
     u0 = varmap_to_vars(u0map,dvs; defaults=defs)
     p = varmap_to_vars(parammap,ps; defaults=defs)
 
-    length(dvs) == length(u0) || throw(ArgumentError("States ($(length(dvs))) and initial conditions ($(length(u0))) are of different lengths."))
+    if u0 !== nothing
+        length(dvs) == length(u0) || throw(ArgumentError("States ($(length(dvs))) and initial conditions ($(length(u0))) are of different lengths."))
+    end
 
     f = constructor(sys,dvs,ps,u0;jac=jac,checkbounds=checkbounds,
                     linenumbers=linenumbers,parallel=parallel,simplify=simplify,