MetaData: restore undocumented set_string behaviour

games/devtest/mods/testmetadata/init.lua

@@ -0,0 +1,36 @@
+
+do
+	-- MetaData test
+	local stack = ItemStack("air 20")
+	local meta = stack:get_meta()
+
+	-- key/value removal using set_string (undocumented, deprecated way)
+	meta:set_string("key", "value")
+	assert(meta:get_string("key") == "value")
+	meta:set_string("key", nil) -- ignore warning
+	assert(meta:to_table().fields["key"] == nil)
+
+	-- undocumented consequence of Lua's automatic string <--> number cast
+	meta:set_string("key", 2)
+	assert(meta:get_string("key") == "2")
+
+	-- Getters for inexistent keys
+	assert(meta:get_string("notfound") == "")
+	assert(meta:get_int("notfound") == 0)
+	assert(meta:get("notfound") == nil)
+
+	-- key/value removal using set_string (correct way)
+	meta:set_string("key", "value2")
+	assert(meta:get_string("key") == "value2")
+	meta:set_string("key", "")
+	assert(meta:to_table().fields["key"] == nil)
+
+	-- from_table
+	local values = meta:to_table()
+	values.fields["new"] = 420
+	meta:from_table(values)
+	assert(meta:get_int("new") == 420)
+	values.fields["new"] = nil
+	meta:from_table(values)
+	assert(meta:get("new") == nil)
+end

games/devtest/mods/testmetadata/mod.conf

@@ -0,0 +1,2 @@
+name = testmetadata
+description = Tests for MetaDataRef

src/script/lua_api/l_metadata.cpp

@@ -115,7 +115,13 @@ int MetaDataRef::l_set_string(lua_State *L)
 
 	MetaDataRef *ref = checkAnyMetadata(L, 1);
 	std::string name = luaL_checkstring(L, 2);
-	auto str = readParam<std::string_view>(L, 3);
+	std::string_view str;
+	if (!lua_isnil(L, 3)) {
+		str = readParam<std::string_view>(L, 3);
+	} else {
+		log_deprecated(L, "Value passed to set_string is nil. This behaviour is"
+			" undocumented and will result in an error in the future.", 1, true);
+	}
 
 	IMetadata *meta = ref->getmeta(!str.empty());
 	if (meta != NULL && meta->setString(name, str))