Fix camera initialization failure on Firefox Android

When VideoConstraints has no constraints, return true.toJS instead of an
empty object. Firefox Android rejects getUserMedia({video: {}}) with an
AbortError, but accepts {video: true}.

This fixes the camera initialization failure in availableCameras() on
Firefox Android while maintaining compatibility with other browsers.

Fixes flutter/flutter#XXXXX

Author: romeo4934

packages/camera/camera_web/lib/src/types/camera_options.dart

@@ -108,12 +108,16 @@ class VideoConstraints {
   // TODO(dit): package:web has a class for this. Use it instead of jsify and toJson.
   /// Convert `this` to something that can be used on the browser.
   JSAny toMediaStreamConstraints() {
-    return <String, Object>{
+    final constraints = <String, Object>{
       if (width != null) 'width': width!.toJson(),
       if (height != null) 'height': height!.toJson(),
       if (facingMode != null) 'facingMode': facingMode!.toJson(),
       if (deviceId != null) 'deviceId': <String, Object>{'exact': deviceId!},
-    }.jsify()!;
+    };
+
+    // Return true instead of empty object for better browser compatibility.
+    // Firefox Android rejects getUserMedia({video: {}}) but accepts {video: true}.
+    return constraints.isEmpty ? true.toJS : constraints.jsify()!;
   }
 
   @override