make it compilable to run `gradle idea`

src/main/java/usi/vaa/elasticsearch/plugin/ingest/lookup/LookupProcessor.java

@@ -32,7 +32,7 @@ public class LookupProcessor extends AbstractProcessor {
 
     public static final String TYPE = "lookup";
 
-    private final String field;
+    private final String field = "";
 
 
     // TODO this signature and method might be incomplete - change as needed
@@ -45,15 +45,18 @@ public class LookupProcessor extends AbstractProcessor {
     public IngestDocument execute(IngestDocument ingestDocument) throws Exception{
         String originalContent = ingestDocument.getFieldValue(field, String.class);
         ingestDocument.setFieldValue(field, "Hello world");
+        return null;
     }
 
     @Override
     public String getType() {
         // TODO your job!
+        return null;
     }
 
     String getField() {
         // TODO your job!
+        return null;
     }
 
 
@@ -66,6 +69,7 @@ public class LookupProcessor extends AbstractProcessor {
             String field = readStringProperty(TYPE, tag, config, "field");
 
             // TODO your job! Also initialize the lookup processor properly if needed, after you changed the constructor
+            return null;
         }
     }
 }