minor follow ups for opaque closure works (#40052)

Author: aviatesk

base/compiler/abstractinterpretation.jl

@@ -1235,7 +1235,7 @@ function abstract_call_opaque_closure(interp::AbstractInterpreter, closure::Part
     pushfirst!(argtypes, closure.env)
     sig = argtypes_to_type(argtypes)
     rt, edgecycle, edge = abstract_call_method(interp, closure.source::Method, sig, Core.svec(), false, sv)
-    add_backedge!(edge, sv)
+    edge !== nothing && add_backedge!(edge, sv)
     tt = closure.typ
     sigT = unwrap_unionall(tt).parameters[1]
     match = MethodMatch(sig, Core.svec(), closure.source::Method, sig <: rewrap_unionall(sigT, tt))

test/opaque_closure.jl

@@ -188,7 +188,7 @@ function oc_varargs_constprop()
     oc = @opaque (args...)->args[1]+args[2]+args[3]
     return Val{oc(1,2,3)}()
 end
-Base.return_types(oc_varargs_constprop, Tuple{}) == Any[Val{6}]
+@test Base.return_types(oc_varargs_constprop, Tuple{}) == Any[Val{6}]
 
 # OpaqueClosure ABI
 f_oc_noinline(x) = @opaque function (y)