place_node crashes the game because of nil player, so add nil checks

* Fix for runtime error in callback `item_OnPlace()` due nill player/placer
* This is for forward preparation on minetest 5.X with backguard compatibility
* related to #39
* related to https://codeberg.org/minenux/minetest-game-adventuretest/issues/39
* related to https://github.com/Bremaweb/adventuretest/issues/39
* update NOTES.md with related fixed about MT 5.X migration

NOTES.md

@@ -3,6 +3,7 @@
 ### migration to mt 5.0/5.2
 
 There are the issues Tignasse Verte mentionned: Adding `placer` in `register_functions.lua` 
+(related commit for minenux are commit a996d5ac709446322d6c6261c21e5f0752a6aeeb )
 and replacing the two occourances of `nodeupdate` with `minetest.check_for_falling`.
 
 Other changes are necessary because I changed (and had to change) quite a lot 

mods/adventuretest/register_functions.lua

@@ -98,7 +98,7 @@ minetest.register_on_dignode(adventuretest_dignode)
 
 local function adventuretest_placenode(pos, node, placer)
   hunger.handle_node_actions(pos,node,placer)
-  if placer:is_player() then
+  if placer and placer:is_player() then
 	  local name = placer:get_player_name()
 	  pd.increment(name,STAT_PLACED,1)
 	  

mods/creative/init.lua

@@ -144,6 +144,7 @@ if minetest.setting_getbool("creative_mode") then
 	})
 	
 	minetest.register_on_placenode(function(pos, newnode, placer, oldnode, itemstack)
+		minetest.log("warning","[creative] maybe  missing check for player here?")
 		return true
 	end)
 	

mods/hud/itemwheel.lua

@@ -2,8 +2,9 @@ local hb = {}
 local scale = tonumber(core.setting_get("hud_scaling")) or 1
 
 local function update_wheel(player)
+
 	local name = player:get_player_name()
-	if not player or not name then
+	if not player or not name or not player:is_player() then
 		return
 	end
 
@@ -169,6 +170,9 @@ minetest.register_on_joinplayer(function(player)
 end)
 
 local function update_wrapper(a, b, player)
+	if not player or not player:is_player() then
+		return
+	end
 	local name = player:get_player_name()
 	if not name then
 		return