Improve AssertUtil for collection checking

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/AssertUtil.java

@@ -15,8 +15,12 @@
  */
 package com.alibaba.csp.sentinel.util;
 
+import java.util.Collection;
+
 /**
  * Util class for checking arguments.
+ *
+ * @author Eric Zhao
  */
 public class AssertUtil {
 
@@ -28,6 +32,12 @@ public class AssertUtil {
         }
     }
 
+    public static void assertNotEmpty(Collection<?> collection, String message) {
+        if (collection == null || collection.isEmpty()) {
+            throw new IllegalArgumentException(message);
+        }
+    }
+
     public static void assertNotBlank(String string, String message) {
         if (StringUtil.isBlank(string)) {
             throw new IllegalArgumentException(message);