Handle race condition in CRT tx pause

If S3MetaRequestPauseObservable#pause is invoked before
S3MetaRequestPauseObservable#subscribe is executed, an NPE can occur.
This can happen if the transfer hasn't actually begin before the pause()
was invoked.

Fix this by returning `null` in this case, which `CrtFileUpload` treats
as transfer not yet started.

Author: dagnir

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/crt/S3MetaRequestPauseObservable.java

@@ -43,7 +43,11 @@ public void subscribe(S3MetaRequestWrapper request) {
      * Pause the request
      */
     public ResumeToken pause() {
-        return pause.apply(request);
+        S3MetaRequestWrapper r = this.request;
+        if (r != null) {
+            return pause.apply(r);
+        }
+        return null;
     }
 }