refactor: improved type handing in wasm engine

frontends/web/ts/gb.ts

@@ -549,7 +549,7 @@ export class GameboyEmulator extends EmulatorBase implements Emulator {
 
     get wasmEngine(): string | null {
         if (!this.gameBoy) return null;
-        return this.gameBoy.get_wasm_engine_ws();
+        return this.gameBoy.get_wasm_engine_ws() ?? null;
     }
 
     get framerate(): number {

src/gb.rs

@@ -322,15 +322,15 @@ impl GameBoy {
         self.ppu().set_palette_colors(&palette);
     }
 
-    pub fn get_wasm_engine_ws(&self) -> String {
+    pub fn get_wasm_engine_ws(&self) -> Option<String> {
         let dependencies = dependencies_map();
         if !dependencies.contains_key("wasm-bindgen") {
-            return String::from("-");
+            return None;
         }
-        String::from(format!(
+        Some(String::from(format!(
             "wasm-bindgen/{}",
             *dependencies.get("wasm-bindgen").unwrap()
-        ))
+        )))
     }
 
     fn js_to_pixel(value: &JsValue) -> Pixel {