Set `allow_empty = True` on `system_python` glob

If there is no local Python installation then the contents of this glob will be
empty. We want the build to succeed in that scenario, so this change will allow
that to happen.

PiperOrigin-RevId: 559407373

bazel/system_python.bzl

@@ -97,7 +97,7 @@ load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
 
 cc_library(
    name = "python_headers",
-   hdrs = glob(["python/**/*.h"]),
+   hdrs = glob(["python/**/*.h"], allow_empty = True),
    includes = ["python"],
    visibility = ["//visibility:public"],
 )