[HUDI-327] Add null/empty checks to key generators (#1040)

* Adds null and empty checks to all key generators. 
* Also improves error messaging for key generator issues.

hudi-common/src/main/java/org/apache/hudi/exception/HoodieKeyException.java

@@ -0,0 +1,35 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hudi.exception;
+
+/**
+ * <p>
+ * Exception thrown for Hoodie Key Generator related errors.
+ * </p>
+ */
+public class HoodieKeyException extends HoodieException {
+
+  public HoodieKeyException(String msg) {
+    super(msg);
+  }
+
+  public HoodieKeyException(String msg, Throwable e) {
+    super(msg, e);
+  }
+}