theme: Change autocomplete value for *_font_fallbacks (#16759)

This PR follows up #16466, changes the default value used when
autocompleting the `ui_font_fallbacks` and `ui_font_fallbacks` settings
from `null` to `[]`.

Special thanks to @maxdeviant for the guidance on writing better code!


Release Notes:

- N/A

crates/theme/src/settings.rs

@@ -247,6 +247,7 @@ pub struct ThemeSettingsContent {
     pub ui_font_family: Option<String>,
     /// The font fallbacks to use for rendering in the UI.
     #[serde(default)]
+    #[schemars(default = "default_font_fallbacks")]
     pub ui_font_fallbacks: Option<Vec<String>>,
     /// The OpenType features to enable for text in the UI.
     #[serde(default)]
@@ -260,6 +261,7 @@ pub struct ThemeSettingsContent {
     pub buffer_font_family: Option<String>,
     /// The font fallbacks to use for rendering in text buffers.
     #[serde(default)]
+    #[schemars(default = "default_font_fallbacks")]
     pub buffer_font_fallbacks: Option<Vec<String>>,
     /// The default font size for rendering in text buffers.
     #[serde(default)]
@@ -299,6 +301,10 @@ fn default_font_features() -> Option<FontFeatures> {
     Some(FontFeatures::default())
 }
 
+fn default_font_fallbacks() -> Option<FontFallbacks> {
+    Some(FontFallbacks::default())
+}
+
 impl ThemeSettingsContent {
     /// Sets the theme for the given appearance to the theme with the specified name.
     pub fn set_theme(&mut self, theme_name: String, appearance: Appearance) {