Make ED_object_modifier_add() accept NULL scene parameter.

This data is only used to get current time/frame value, which is never
mandatory to add a modifier.

Needed by incoming fix to support particles modifiers in liboverrides.

source/blender/blenkernel/intern/particle.c

@@ -3565,7 +3565,9 @@ ModifierData *object_add_particle_system(Main *bmain, Scene *scene, Object *ob,
 
   psys->totpart = 0;
   psys->flag = PSYS_CURRENT;
-  psys->cfra = BKE_scene_frame_to_ctime(scene, CFRA + 1);
+  if (scene != NULL) {
+    psys->cfra = BKE_scene_frame_to_ctime(scene, CFRA + 1);
+  }
 
   DEG_relations_tag_update(bmain);
   DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);

source/blender/blenkernel/intern/softbody.c

@@ -3132,8 +3132,10 @@ SoftBody *sbNew(Scene *scene)
   sb->inpush = 0.5f;
 
   sb->interval = 10;
-  sb->sfra = scene->r.sfra;
-  sb->efra = scene->r.efra;
+  if (scene != NULL) {
+    sb->sfra = scene->r.sfra;
+    sb->efra = scene->r.efra;
+  }
 
   sb->colball = 0.49f;
   sb->balldamp = 0.50f;

source/blender/editors/object/object_modifier.c

@@ -127,6 +127,11 @@ static void object_force_modifier_bind_simple_options(Depsgraph *depsgraph,
   md_eval->mode = mode;
 }
 
+/** Add a modifier to given object, including relevant extra processing needed by some physics
+ * types (particles, simulations...).
+ *
+ * \param scene is only used to set current frame in some cases, and may be NULL.
+ */
 ModifierData *ED_object_modifier_add(
     ReportList *reports, Main *bmain, Scene *scene, Object *ob, const char *name, int type)
 {