lib/nbt: Make list serielizer return an error when serializing a list with no contained type set

src/lib/nbt.cpp

@@ -868,6 +868,9 @@ namespace NBT {
         }
 
         ErrorOrVoid List::serializeWithoutHeader(std::vector<uint8_t>* rawData) {
+            if (this->containedType == TagType::INVALID) {
+                return ErrorOrVoid(true, ErrorCodes::INVALID_TYPE);
+            }
             rawData->push_back(this->containedType);
 
             // 32 bit signed integer max