Fix rare server crash from hack clients trying to connect (Cloudburst…

…MC#1881)

(cherry picked from commit 51cc75d)

src/main/java/cn/nukkit/Server.java

@@ -2201,7 +2201,7 @@ public boolean isWhitelisted(String name) {
     }
 
     public boolean isOp(String name) {
-        return this.operators.exists(name, true);
+        return name != null && this.operators.exists(name, true);
     }
 
     public Config getWhitelist() {