extensions: Fix language detection (#968)

Author: Diego-Ivan

src/Extensions/Extensions.js

@@ -53,22 +53,22 @@ export function Extensions({ window }) {
   window.add_action(action_extensions);
 }
 
-let rust_enabled = false;
+let rust_enabled = null;
 export function isRustEnabled() {
   rust_enabled ??=
     Gio.File.new_for_path("/usr/lib/sdk/rust-stable").query_exists(null) &&
     Gio.File.new_for_path(`/usr/lib/sdk/${llvm}`).query_exists(null);
   return rust_enabled;
 }
 
-let vala_enabled = false;
+let vala_enabled = null;
 export function isValaEnabled() {
   vala_enabled ??=
     Gio.File.new_for_path("/usr/lib/sdk/vala").query_exists(null);
   return vala_enabled;
 }
 
-let typescript_enabled = false;
+let typescript_enabled = null;
 export function isTypeScriptEnabled() {
   typescript_enabled ??=
     Gio.File.new_for_path("/usr/lib/sdk/typescript").query_exists(null) &&