fix: Handles empty variables query parameter

Sources/GraphQLHummingbird/HTTP/GraphQLHandler+HTTP.swift

@@ -6,13 +6,39 @@ import NIOCore
 
 extension GraphQLHandler {
     /// https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#get
-    func handleGet(request: Request, context: Context) async throws -> Response {
+    func handleGet(request: Request, context: Context, jsonDecoder: JSONDecoder) async throws
+        -> Response
+    {
         guard config.allowGet else {
             throw HTTPError(.methodNotAllowed, message: "GET requests are disallowed")
         }
 
         // Decode query parameters as GraphQLRequest
-        let graphQLRequest = try request.uri.decodeQuery(as: GraphQLRequest.self, context: context)
+        let queryParameters = request.uri.queryParameters
+        guard let query = queryParameters.get("query") else {
+            throw HTTPError(.badRequest, message: "`query` parameter is required")
+        }
+        let variables: [String: Map]
+        if let queryVariables = queryParameters.get("variables") {
+            do {
+                variables = try jsonDecoder.decode(
+                    [String: Map].self,
+                    from: Data(queryVariables.utf8)
+                )
+            } catch {
+                throw HTTPError(
+                    .badRequest,
+                    message: "`variable` parameter could not be decoded: \(error)"
+                )
+            }
+        } else {
+            variables = [:]
+        }
+        let graphQLRequest = GraphQLRequest(
+            query: query,
+            operationName: queryParameters.get("operationName"),
+            variables: variables
+        )
 
         let operationType: OperationType
         do {

Sources/GraphQLHummingbird/Router+graphql.swift

@@ -53,7 +53,11 @@ extension RouterMethods {
             }
 
             // Normal GET request handling
-            return try await handler.handleGet(request: request, context: context)
+            return try await handler.handleGet(
+                request: request,
+                context: context,
+                jsonDecoder: config.coders.jsonDecoder
+            )
         }
 
         post(path) { request, context in

Tests/GraphQLHummingbirdTests/HTTPTests.swift

@@ -272,6 +272,29 @@ struct HTTPTests {
         }
     }
 
+    @Test func getRequestParsesEmtpyVariables() async throws {
+        let router = Router()
+        router.graphql(schema: helloWorldSchema) { _ in
+            EmptyContext()
+        }
+        let app = Application(router: router)
+
+        try await app.test(.router) { client in
+            try await client.execute(
+                uri: "/graphql?query=%7Bhello%7D&variables=%7B%7D",
+                method: .get,
+                headers: jsonGraphQLHeaders
+            ) { response in
+                var body = response.body
+                #expect(response.status == .ok)
+
+                let result = try defaultJSONDecoder.decode(GraphQLResult.self, from: response.body)
+                #expect(result.data?["hello"] == "World")
+                #expect(result.errors.isEmpty)
+            }
+        }
+    }
+
     @Test func disallowGetRequest() async throws {
         let router = Router()
         router.graphql(