[fix](multicatalog) fix metacache is null issue when show_dbs (apache…

…#38619)

Brought by: apache#38244

how to reproduce:
create a hms catalog named hive with property use_meta_cache = false,
after that restart fe, and run sql `switch hive; show databases;`, will
get error：
```sql
ERROR 1105 (HY000): NullPointerException, msg: java.lang.NullPointerException: Cannot invoke "org.apache.doris.datasource.metacache.MetaCache.listNames()" because "this.metaCache" is null
```

reason, property use_meta_cache of newly created hms catalog after pr
apache#38244 was force convert to true, but field `useMetaCache` of class
ExternalCatalog is not converted at the same time.

fe/fe-core/src/main/java/org/apache/doris/datasource/ExternalCatalog.java

@@ -294,6 +294,7 @@ public void checkProperties() throws DdlException {
         if (properties.getOrDefault(ExternalCatalog.USE_META_CACHE, "true").equals("false")) {
             LOG.warn("force to set use_meta_cache to true for catalog: {} when creating", name);
             getCatalogProperty().addProperty(ExternalCatalog.USE_META_CACHE, "true");
+            useMetaCache = Optional.of(true);
         }
     }