Fix core.get_node_or_nil in emerge env

"ignore" does not mean unloaded, we have to properly check it.

builtin/emerge/env.lua

@@ -31,11 +31,6 @@ function core.get_node(pos)
 	return core.vmanip:get_node_at(pos)
 end
 
-function core.get_node_or_nil(pos)
-	local node = core.vmanip:get_node_at(pos)
-	return node.name ~= "ignore" and node
-end
-
 function core.get_perlin(seed, octaves, persist, spread)
 	local params
 	if type(seed) == "table" then

src/script/lua_api/l_env.cpp

@@ -1534,6 +1534,19 @@ void ModApiEnv::InitializeClient(lua_State *L, int top)
 	if (!vm)                     \
 		return 0
 
+// get_node_or_nil(pos)
+int ModApiEnvVM::l_get_node_or_nil(lua_State *L)
+{
+	GET_VM_PTR;
+
+	v3s16 pos = read_v3s16(L, 1);
+	if (vm->exists(pos))
+		pushnode(L, vm->getNodeRefUnsafe(pos));
+	else
+		lua_pushnil(L);
+	return 1;
+}
+
 // get_node_max_level(pos)
 int ModApiEnvVM::l_get_node_max_level(lua_State *L)
 {
@@ -1703,6 +1716,7 @@ MMVManip *ModApiEnvVM::getVManip(lua_State *L)
 void ModApiEnvVM::InitializeEmerge(lua_State *L, int top)
 {
 	// other, more trivial functions are in builtin/emerge/env.lua
+	API_FCT(get_node_or_nil);
 	API_FCT(get_node_max_level);
 	API_FCT(get_node_level);
 	API_FCT(set_node_level);

src/script/lua_api/l_env.h

@@ -247,6 +247,9 @@ class ModApiEnv : public ModApiEnvBase {
 class ModApiEnvVM : public ModApiEnvBase {
 private:
 
+	// get_node_or_nil(pos)
+	static int l_get_node_or_nil(lua_State *L);
+
 	// get_node_max_level(pos)
 	static int l_get_node_max_level(lua_State *L);