use empty string for repo env due to bazel 7

Author: rickeylev

python/private/runtime_env_repo.bzl

@@ -10,6 +10,10 @@ def _runtime_env_repo_impl(rctx):
             op = "GetPyenvVersionFile",
             arguments = [pyenv, "version-file"],
         ).stdout.strip()
+
+        # When pyenv is used, the version file is what decided the
+        # version used. Watch it so we compute the correct value if the
+        # user changes it.
         rctx.watch(pyenv_version_file)
 
     version = repo_utils.execute_checked(
@@ -24,7 +28,9 @@ def _runtime_env_repo_impl(rctx):
         environment = {
             # Prevent the user's current shell from influencing the result.
             # This envvar won't be present when a test is run.
-            "PYENV_VERSION": None,
+            # NOTE: This should be None, but Bazel 7 doesn't support None
+            # values. Thankfully, pyenv treats empty string the same as missing.
+            "PYENV_VERSION": "",
         },
     ).stdout.strip()
     rctx.file("info.bzl", "PYTHON_VERSION = '{}'\n".format(version))