fix bug

mybatis-multi-tenancy/src/main/java/com/iformall/plugin/shard/plugin/ShardingSpherePlugin.java

@@ -178,7 +178,10 @@ public class ShardingSpherePlugin implements MyBatisPlus {
     
     private Object getValue(String column,Object parameterObject,Map<String,Object> parentChainMap) {
     	if (null != parentChainMap && parentChainMap.containsKey(column)) {
-    		return parentChainMap.get(column);
+    		Object o =  parentChainMap.get(column);
+    		if (null != o) {
+    			return o;
+    		}
     	}
     	
     	if (parameterObject instanceof Map) {