Fix copy/paste in the Linux embedding (#18624)

The recent refactoring to support headless mode accidentally passed
nullptr instead of the window (if any) to the platform handler that
manages clipboard interactions, causing it to be broken.

Fixes https://github.com/flutter/flutter/issues/58035
https://dart.googlesource.com/external/github.com/flutter/engine/+/a03c752a81e310621897b8f6a3cd634e2429b711
diff --git a/commits.json b/commits.json
index 6488c9c..217b55e 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"339e1607e78c69fd6452453c89dc1ba19fcbeeb2",
+  "external/github.com/flutter/engine":"a03c752a81e310621897b8f6a3cd634e2429b711",
   "external/github.com/flutter/flutter":"5663e02645a6e69fedfe8553cbe7940cbb592be9",
   "sdk":"7a63d5889cb413584188f9a73eb88778d81d0ab3"
 }
\ No newline at end of file