🎨 修复代码，避免 String 与 Integer 强转报错

Author: ChinaTjcm

weixin-java-common/src/main/java/me/chanjar/weixin/common/util/xml/IntegerArrayConverter.java

@@ -24,6 +24,11 @@ public String toString(Object obj) {
 
   @Override
   public Object fromString(String str) {
+
+    if (str == null || str.length() == 0) {
+      return null;
+    }
+
     final Iterable<String> iterable = Splitter.on(",").split(str);
     final String[] strings = Iterables.toArray(iterable, String.class);
     Integer[] result = new Integer[strings.length];