[producer] Allow deleting records via the shell producer (#646)

The shell producer can allow deleting records by accepting the value null regardless of the fact that it is compatible with the store schema or not. Note that other forms of null like "null", 'null', etc are not allowed.

clients/venice-producer/src/main/java/com/linkedin/venice/producer/online/ProducerTool.java

@@ -184,10 +184,14 @@ private static void writeToStore(ProducerContext producerContext) throws Excepti
       RouterBasedStoreSchemaFetcher schemaFetcher = new RouterBasedStoreSchemaFetcher(castClient);
       Schema keySchema = schemaFetcher.getKeySchema();
       Object key = adaptDataToSchema(producerContext.key, keySchema);
-      Object value = getValueObject(producerContext.value, schemaFetcher);
-
-      producer.asyncPut(key, value).get();
-      System.out.println("Data written to Venice!");
+      if (producerContext.value.equals("null")) { // Only allow `null`. Not "null", or 'null', or whatever.
+        producer.asyncDelete(key).get();
+        System.out.println("Record deleted from Venice!");
+      } else {
+        Object value = getValueObject(producerContext.value, schemaFetcher);
+        producer.asyncPut(key, value).get();
+        System.out.println("Data written to Venice!");
+      }
     } catch (Exception e) {
       System.err.println(ExceptionUtils.stackTraceToString(e));
       System.exit(1);