Add missing serde default

src/config.rs

@@ -147,6 +147,7 @@ pub struct StateConfig {
     /// The language tag of the HTML file.
     #[serde(default = "default_lang")]
     pub lang: String,
+    #[serde(default)]
     pub strings: Strings,
 }
 fn default_path_prefix() -> String {
@@ -163,6 +164,7 @@ pub struct Config {
     #[serde(default = "default_socket_address")]
     pub socket_address: String,
     pub email: Email,
+    #[serde(default)]
     pub utc_offset: UtcOffset,
     #[serde(flatten)]
     pub state_config: StateConfig,