fix: use default disk.columns when only other disk.* configs exist (#1776)

I configured `[disk.name_filter]` and after upgrading to v0.11.0, the
disk widget became empty since `disk.columns` was parsed as an empty
vector unless I added `disk.columns` to my config as well.

src/options.rs

@@ -407,7 +407,7 @@ pub(crate) fn init_app(args: BottomArgs, config: Config) -> Result<(App, BottomL
                                 DiskTableWidget::new(
                                     &app_config_fields,
                                     &styling,
-                                    config.disk.as_ref().map(|cfg| cfg.columns.as_slice()),
+                                    config.disk.as_ref().and_then(|cfg| cfg.columns.as_deref()),
                                 ),
                             );
                         }

src/options/config/disk.rs

@@ -16,7 +16,7 @@ pub(crate) struct DiskConfig {
 
     /// A list of disk widget columns.
     #[serde(default)]
-    pub(crate) columns: Vec<DiskColumn>, // TODO: make this more composable(?) in the future, we might need to rethink how it's done for custom widgets
+    pub(crate) columns: Option<Vec<DiskColumn>>, // TODO: make this more composable(?) in the future, we might need to rethink how it's done for custom widgets
 }
 
 #[cfg(test)]
@@ -24,10 +24,17 @@ mod test {
     use super::DiskConfig;
 
     #[test]
-    fn empty_column_setting() {
+    fn none_column_setting() {
         let config = "";
         let generated: DiskConfig = toml_edit::de::from_str(config).unwrap();
-        assert!(generated.columns.is_empty());
+        assert!(generated.columns.is_none());
+    }
+
+    #[test]
+    fn empty_column_setting() {
+        let config = r#"columns = []"#;
+        let generated: DiskConfig = toml_edit::de::from_str(config).unwrap();
+        assert!(generated.columns.unwrap().is_empty());
     }
 
     #[test]