patch 8.2.3035: Vim9: crash when calling :def function with partial

Problem:    Vim9: crash when calling :def function with partial and return
            type is not set.
Solution:   When the return type is not set handle like the return type is
            unknown. (closes #8422)

src/testdir/test_vim9_func.vim

@@ -1005,6 +1005,20 @@ def Test_pass_legacy_lambda_to_def_func()
       Foo()
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      def g:TestFunc(f: func())
+      enddef
+      legacy call g:TestFunc({-> 0})
+      delfunc g:TestFunc
+
+      def g:TestFunc(f: func(number))
+      enddef
+      legacy call g:TestFunc({nr -> 0})
+      delfunc g:TestFunc
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 " Default arg and varargs

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3035,
 /**/
     3034,
 /**/

src/vim9type.c

@@ -171,7 +171,7 @@ alloc_func_type(type_T *ret_type, int argcount, garray_T *type_gap)
     if (type == NULL)
 	return &t_any;
     type->tt_type = VAR_FUNC;
-    type->tt_member = ret_type;
+    type->tt_member = ret_type == NULL ? &t_unknown : ret_type;
     type->tt_argcount = argcount;
     type->tt_args = NULL;
     return type;
@@ -188,7 +188,7 @@ get_func_type(type_T *ret_type, int argcount, garray_T *type_gap)
     // recognize commonly used types
     if (argcount <= 0)
     {
-	if (ret_type == &t_unknown)
+	if (ret_type == &t_unknown || ret_type == NULL)
 	{
 	    // (argcount == 0) is not possible
 	    return &t_func_unknown;