fix(sqllab): Allow clear on schema and catalog (#32515)

(cherry picked from commit 4c3aae7)

Author: justinpark

superset-frontend/src/components/DatabaseSelector/index.tsx

@@ -392,6 +392,7 @@ export default function DatabaseSelector({
         options={catalogOptions}
         showSearch
         value={currentCatalog || undefined}
+        allowClear
       />,
       refreshIcon,
     );
@@ -418,6 +419,7 @@ export default function DatabaseSelector({
         options={schemaOptions}
         showSearch
         value={currentSchema}
+        allowClear
       />,
       refreshIcon,
     );