Handle backport PRs without piper info in release notes automation

Example commits on v1.67.x that are failing the current regex match:

```
commit 1eb5673
Author: apolcyn <[email protected]>
Date:   Mon Sep 16 12:35:34 2024 -0700

    [objc] backport grpc#37690 to v1.67.x (grpc#37712)

    Backport grpc#37690

    cc @HannahShiSFB @sampajano

    Co-authored-by: Hannah Shi <[email protected]>

commit ace22e3
Author: apolcyn <[email protected]>
Date:   Thu Sep 12 10:16:46 2024 -0700

    [ruby] reduce an INFO log to DEBUG (backport grpc#37633) (grpc#37686)

    Backport grpc#37633 to 1.67

    Note grpc#37633 is not yet merged b/c of
    the master branch freeze. Merging on 1.67 with the idea that
    grpc#37633 will merge as soon as the master
    branch is unfrozen.
```

PiperOrigin-RevId: 681960402

Author: apolcyn

tools/release/release_notes.py

@@ -104,15 +104,16 @@ def get_commit_detail(commit):
     detail += "- " + title
     if not title.endswith("."):
         detail += "."
-    matches = re.search("PiperOrigin-RevId: ([0-9]+)$", output)
-    cl_num = matches.group(1)
-    detail += (
-        " ([commit](https://github.com/grpc/grpc/commit/"
-        + commit
-        + ")) ([CL](https://critique.corp.google.com/cl/"
-        + cl_num
-        + "))"
+    detail += " ([commit](https://github.com/grpc/grpc/commit/{}))".format(
+        commit
     )
+    matches = re.search("PiperOrigin-RevId: ([0-9]+)$", output)
+    # backport commits might not have PiperOrigin-RevId
+    if matches is not None:
+        cl_num = matches.group(1)
+        detail += " ([CL](https://critique.corp.google.com/cl/{}))".format(
+            cl_num
+        )
     return detail