[HUDI-143] Excluding javax.* from utilities and spark bundles

- Plus minor code review comments

hudi-common/src/main/java/org/apache/hudi/common/util/StringUtils.java

@@ -62,7 +62,7 @@ public class StringUtils {
     return sb.toString();
   }
 
-  public static boolean isEmpty(String str) {
+  public static boolean isNullOrEmpty(String str) {
     return str == null || str.length() == 0;
   }
 }