[fix](unique key) getEnableUniqueKeyMergeOnWrite should check keys ty…

…pe (apache#44692)

Problem Summary:

Some user can create Duplicate keys table with
`enable_unique_key_merge_on_write=true` in version 1.2, when they
upgrade to version 2.0, some logic in BE treat such table as an unique
key mow table, make a wrong query plan, which cause BE core.

In the method `OlapTable::getEnableUniqueKeyMergeOnWrite()`, we should
check if the table is a unique key table.

fe/fe-core/src/main/java/org/apache/doris/catalog/OlapTable.java

@@ -2713,6 +2713,9 @@ public boolean getEnableUniqueKeyMergeOnWrite() {
         if (tableProperty == null) {
             return false;
         }
+        if (getKeysType() != KeysType.UNIQUE_KEYS) {
+            return false;
+        }
         return tableProperty.getEnableUniqueKeyMergeOnWrite();
     }