fix(sigv4): Convert empty query parameters to null (#6082)

Author: tyllark

packages/aws_signature_v4/lib/src/signer/aws_signer.dart

@@ -256,7 +256,8 @@ class AWSSigV4Signer {
   }) {
     // The signing process requires component keys be encoded. However, the
     // actual HTTP request should have the pre-encoded keys.
-    final queryParameters = Map.of(canonicalRequest.queryParameters);
+    Map<String, String>? queryParameters =
+        Map.of(canonicalRequest.queryParameters);
 
     // Similar to query parameters, some header values are canonicalized for
     // signing. However their original values should be included in the
@@ -284,6 +285,13 @@ class AWSSigV4Signer {
       }
     }
 
+    // Web sends an OPTIONS request to verify CORS compatibility with the URL.
+    // A 404 can be returned if the URL contains unexpected query Parameters
+    // and URI.toString() appends a "?" to the URL for an empty query parameter
+    // map. Set the query parameter to null if it empty to avoid this.
+    // https://github.com/dart-lang/sdk/issues/51656
+    queryParameters = queryParameters.isEmpty ? null : queryParameters;
+
     // On Web, sign the `Host` and `Content-Length` headers, but do not send
     // them as part of the request, since these will be included automatically
     // by the browser and most now restrict the ability to set them via code.