[fix](common) If the properties in DDL is not provided, the mysql client will lost connection (#30256)

fe/fe-core/src/main/java/org/apache/doris/common/util/PrintableMap.java

@@ -93,6 +93,9 @@ public class PrintableMap<K, V> {
 
     @Override
     public String toString() {
+        if (map == null) {
+            return "";
+        }
         StringBuilder sb = new StringBuilder();
         Iterator<Map.Entry<K, V>> iter = showEntries().iterator();
         while (iter.hasNext()) {