Be explicit about alowing empty globs

There are several globs that are empty and this prevents building
with the incompatible flag #8195.
This commit just makes it explicit that empty is allowed.

tools/build_defs/pkg/BUILD

@@ -31,7 +31,10 @@ py_library(
 
 filegroup(
  name = "archive_testdata",
- srcs = glob(["testdata/**"]),
+ srcs = glob(
+ ["testdata/**"],
+ allow_empty = True,
+ ),
 )
 
 py_test(