优化代码（NPE）

src/main/java/cn/quantgroup/xyqb/util/PasswordUtil.java

@@ -2,8 +2,10 @@ package cn.quantgroup.xyqb.util;
 
 import cn.quantgroup.xyqb.Constants;
 import com.google.common.base.Preconditions;
+import org.apache.commons.lang3.StringUtils;
 
 import java.security.MessageDigest;
+import java.util.Objects;
 import java.util.concurrent.ThreadLocalRandom;
 
 /**
@@ -22,10 +24,16 @@ public class PasswordUtil {
     };
 
     public final static String MD5WithSalt(String s){
+        if(Objects.isNull(s)){
+            return null;
+        }
         return MD5(s.toLowerCase() + Constants.PASSWORD_SALT);
     }
 
     public final static String MD5(String s) {
+        if(Objects.isNull(s)){
+            return null;
+        }
         try {
             byte[] strTemp = s.getBytes("utf-8");
             MessageDigest mdTemp = MessageDigest.getInstance("MD5");