feat: improve slow queries options deserialization (#6734)

* feat: improve slow queries options deserialization

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

* refactor: use serde default for struct

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

---------

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

src/common/base/src/lib.rs

@@ -20,6 +20,7 @@ pub mod range_read;
 #[allow(clippy::all)]
 pub mod readable_size;
 pub mod secrets;
+pub mod serde;
 
 pub type AffectedRows = usize;
 

src/common/base/src/serde.rs

@@ -0,0 +1,31 @@
+// Copyright 2023 Greptime Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use serde::{Deserialize, Deserializer};
+
+/// Deserialize an empty string as the default value.
+pub fn empty_string_as_default<'de, D, T>(deserializer: D) -> Result<T, D::Error>
+where
+    D: Deserializer<'de>,
+    T: Default + Deserialize<'de>,
+{
+    let s = String::deserialize(deserializer)?;
+
+    if s.is_empty() {
+        Ok(T::default())
+    } else {
+        T::deserialize(serde::de::value::StringDeserializer::<D::Error>::new(s))
+            .map_err(serde::de::Error::custom)
+    }
+}