GetEnvVariable: return std::optional<std::string> (#1017)

- `GetEnvVariable` now returns `std::optional<std::string>` so callers can distinguish an unset variable from one set to an empty value.
- Windows path uses `SetLastError(ERROR_SUCCESS)` + `ERROR_ENVVAR_NOT_FOUND` to detect "not found"; POSIX path returns `nullopt` when `getenv` returns `nullptr`.
- All call sites migrated. Most use `.value_or("")` to preserve current empty-or-unset behavior. The diagnostic helpers in `zen-test/artifactprovider-tests.cpp` now report `<unset>` vs `<empty>` distinctly.
- Added a check in the `ExpandEnvironmentVariables` test confirming `nullopt` for an unset variable; PATH/HOME lookups in that test use `REQUIRE(has_value())` so a missing var fails cleanly instead of throwing `bad_optional_access`.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/zen/authutils.cpp b/src/zen/authutils.cpp
index a2af2b63e..f186a2b0b 100644
--- a/src/zen/authutils.cpp
+++ b/src/zen/authutils.cpp
@@ -338,7 +338,7 @@ AuthCommandLineOptions::ParseOptions(cxxopts::Options&			  Ops,
 	auto GetEnvAccessToken = [](const std::string& AccessTokenEnv) -> std::string {
 		if (!AccessTokenEnv.empty())
 		{
-			return GetEnvVariable(AccessTokenEnv);
+			return GetEnvVariable(AccessTokenEnv).value_or("");
 		}
 		return {};
 	};