Handle missing login in changelog generate script

Noticed during apache#1822 that the script initially failed
due to [this
commit](apache@10cf7c1)
no longer having the associated GitHub user.

Updates the script to skip the username referencing in such cases.

Author: iffyio

dev/release/generate-changelog.py

@@ -28,7 +28,8 @@ def print_pulls(repo_name, title, pulls):
         print()
         for (pull, commit) in pulls:
             url = "https://github.com/{}/pull/{}".format(repo_name, pull.number)
-            print("- {} [#{}]({}) ({})".format(pull.title, pull.number, url, commit.author.login))
+            author = f"({commit.author.login})" if commit.author else ''
+            print("- {} [#{}]({}) {}".format(pull.title, pull.number, url, author))
         print()
 
 
@@ -161,4 +162,4 @@ def cli(args=None):
     generate_changelog(repo, project, args.tag1, args.tag2, args.version)
 
 if __name__ == "__main__":
-    cli()
+    cli()