[webview_flutter_wkwebview] Fix testNilRequest test for macOS 15.5 (#9330)

This test was introduced in #8766 to fix a crash when `WKFrameInfo.request` returned nil. It appears that macOS 15.5 fixed `WKFrameInfo.request` so it no longer returns nil and instead returns an empty `URLRequest`. This updates the test to accept either `nil` or an empty `URLRequest`.

Fixes flutter/flutter#169511.

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Author: vashworth

packages/webview_flutter/webview_flutter_wkwebview/darwin/Tests/FrameInfoProxyAPITests.swift

@@ -35,8 +35,13 @@ class FrameInfoProxyAPITests: XCTestCase {
 
     let instance = TestFrameInfoWithNilRequest()
     let value = try? api.pigeonDelegate.request(pigeonApi: api, pigeonInstance: instance)
-
-    XCTAssertNil(value)
+    // On macOS 15.5+, `WKFrameInfo.request` returns with an empty URLRequest.
+    // Previously it would return nil so accept either.
+    if value != nil {
+      XCTAssertEqual(value?.value.url?.absoluteString, "")
+    } else {
+      XCTAssertNil(value)
+    }
   }
 }