Fix incompatibility with Bazel 6 (#248)

Adds a missing `None` to an otherwise failing `getattr`.
diff --git a/fuzzing/private/binary.bzl b/fuzzing/private/binary.bzl
index 4c85aed..8ff9723 100644
--- a/fuzzing/private/binary.bzl
+++ b/fuzzing/private/binary.bzl
@@ -114,7 +114,7 @@
     else:
         default_info = ctx.attr.binary[DefaultInfo]
     binary_runfiles = default_info.default_runfiles
-    binary_repo_mapping_manifest = getattr(default_info.files_to_run, "repo_mapping_manifest")
+    binary_repo_mapping_manifest = getattr(default_info.files_to_run, "repo_mapping_manifest", None)
     other_runfiles = []
     if ctx.file.corpus:
         other_runfiles.append(ctx.file.corpus)