Add isNullOrEmpty

hippo4j-common/src/main/java/cn/hippo4j/common/toolkit/StringUtil.java

@@ -46,6 +46,16 @@ public class StringUtil {
         return str == null ? "" : str;
     }
 
+    /**
+     * Returns {@code true} if the given string is null or is the empty string.
+     *
+     * @param str
+     * @return
+     */
+    public static boolean isNullOrEmpty(String str) {
+        return str == null || str.isEmpty();
+    }
+
     /**
      * Is blank.
      *