settings_ui: Add maybe settings (#40724)

Closes #ISSUE

Adds a `Maybe<T>` type to `settings_content`, that makes the distinction
between `null` and omitted settings values explicit. This unlocks a few
more settings in the settings UI

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/settings/src/settings_content/project.rs

@@ -8,7 +8,7 @@ use settings_macros::MergeFrom;
 use util::serde::default_true;
 
 use crate::{
-    AllLanguageSettingsContent, DelayMs, ExtendingVec, ProjectTerminalSettingsContent,
+    AllLanguageSettingsContent, DelayMs, ExtendingVec, Maybe, ProjectTerminalSettingsContent,
     SlashCommandSettings,
 };
 
@@ -56,11 +56,13 @@ pub struct ProjectSettingsContent {
 #[skip_serializing_none]
 #[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize, JsonSchema, MergeFrom)]
 pub struct WorktreeSettingsContent {
-    /// The displayed name of this project. If not set or empty, the root directory name
+    /// The displayed name of this project. If not set or null, the root directory name
     /// will be displayed.
     ///
-    /// Default: ""
-    pub project_name: Option<String>,
+    /// Default: null
+    #[serde(default)]
+    #[serde(skip_serializing_if = "Maybe::is_unset")]
+    pub project_name: Maybe<String>,
 
     /// Completely ignore files matching globs from `file_scan_exclusions`. Overrides
     /// `file_scan_inclusions`.

crates/settings/src/settings_content/terminal.rs

@@ -134,7 +134,19 @@ pub struct TerminalSettingsContent {
 }
 
 /// Shell configuration to open the terminal with.
-#[derive(Clone, Debug, Default, Serialize, Deserialize, PartialEq, Eq, JsonSchema, MergeFrom)]
+#[derive(
+    Clone,
+    Debug,
+    Default,
+    Serialize,
+    Deserialize,
+    PartialEq,
+    Eq,
+    JsonSchema,
+    MergeFrom,
+    strum::EnumDiscriminants,
+)]
+#[strum_discriminants(derive(strum::VariantArray, strum::VariantNames, strum::FromRepr))]
 #[serde(rename_all = "snake_case")]
 pub enum Shell {
     /// Use the system's default terminal configuration in /etc/passwd