dawn_node: Fix default adapter selection

The Null adapter was always being used when no backend override was
specified, instead of the platform's default adapter.

Change-Id: I3d8ae62aacda1309a141c3ca39d6e03252fc5e94
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/75700
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/dawn_node/binding/GPU.cpp

@@ -136,6 +136,7 @@ namespace wgpu::binding {
                 promise.Reject("unknown backend '" + forceBackend + "'");
                 return promise;
             }
+        }
 
         bool found = false;
         for (size_t i = 0; i < adapters.size(); ++i) {
@@ -148,9 +149,12 @@ namespace wgpu::binding {
             }
         }
         if (!found) {
+            if (!forceBackend.empty()) {
                 promise.Reject("backend '" + forceBackend + "' not found");
-                return promise;
+            } else {
+                promise.Reject("no suitable backends found");
             }
+            return promise;
         }
 
         auto adapter = GPUAdapter::Create<GPUAdapter>(env, adapters[adapterIndex], flags_);