Fix compiling errors

Author: thecatcore

src/main/java/io/github/fabriccompatibilitylayers/modremappingapi/impl/DefaultMappingsConfig.java

@@ -1,6 +1,7 @@
 package io.github.fabriccompatibilitylayers.modremappingapi.impl;
 
 import io.github.fabriccompatibilitylayers.modremappingapi.api.v2.MappingsConfig;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -21,4 +22,9 @@ public Supplier<String> getExtraMappings() {
     public Map<String, String> getRenamingMap() {
         return new HashMap<>();
     }
+
+    @Override
+    public @Nullable String getDefaultPackage() {
+        return null;
+    }
 }