{RDBMS} Return default API version if available in _resolve_api_version function

src/azure-cli/azure/cli/command_modules/rdbms/_flexible_server_util.py

@@ -306,6 +306,8 @@ def _resolve_api_version(client, provider_namespace, resource_type, parent_path)
           if t.resource_type.lower() == resource_type_str.lower()]
     if not rt:
         raise InvalidArgumentValueError('Resource type {} not found.'.format(resource_type_str))
+    if len(rt) == 1 and rt[0].default_api_version not in (None, ''):
+        return rt[0].default_api_version
     if len(rt) == 1 and rt[0].api_versions:
         npv = [v for v in rt[0].api_versions if 'preview' not in v.lower()]
         return npv[0] if npv else rt[0].api_versions[0]