Fix #21066: particle hair X mirror doesn't work.

source/blender/editors/physics/particle_edit.c

@@ -831,9 +831,12 @@ static void PE_apply_mirror(Object *ob, ParticleSystem *psys)
 	edit= psys->edit;
 	psmd= psys_get_modifier(ob, psys);
 
-	if(!edit->mirror_cache || !psmd->dm)
+	if(!psmd->dm)
 		return;
 
+	if(!edit->mirror_cache)
+		PE_update_mirror_cache(ob, psys);
+
 	/* we delay settings the PARS_EDIT_RECALC for mirrored particles
 	 * to avoid doing mirror twice */
 	LOOP_POINTS {