Use validated channel in getNewPluginChannelId

Fixes #2707

common/src/main/java/com/viaversion/viaversion/protocols/protocol1_13to1_12_2/data/MappingData.java

@@ -139,9 +139,9 @@ public class MappingData extends MappingDataBase {
         int separatorIndex = newId.indexOf(':');
         // Vanilla parses an empty and a missing namespace as the minecraft namespace
         if (separatorIndex == -1) {
-            newId = "minecraft:" + newId;
+            return "minecraft:" + newId;
         } else if (separatorIndex == 0) {
-            newId = "minecraft" + newId;
+            return "minecraft" + newId;
         }
         return newId;
     }

common/src/main/java/com/viaversion/viaversion/protocols/protocol1_13to1_12_2/packets/InventoryPackets.java

@@ -483,7 +483,7 @@ public class InventoryPackets extends ItemRewriter<Protocol1_13To1_12_2> {
             default:
                 String mappedChannel = Protocol1_13To1_12_2.MAPPINGS.getChannelMappings().get(old);
                 if (mappedChannel != null) return mappedChannel;
-                return MappingData.isValid1_13Channel(old) ? old : null;
+                return MappingData.validateNewChannel(old);
         }
     }