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/zenutil/authutils.cpp b/src/zenutil/authutils.cpp
index 8ee6b1417..3945c1c62 100644
--- a/src/zenutil/authutils.cpp
+++ b/src/zenutil/authutils.cpp
@@ -24,7 +24,7 @@ FindOidcTokenExePath(std::string_view OidcTokenAuthExecutablePath)
 {
 	if (OidcTokenAuthExecutablePath.empty())
 	{
-		std::filesystem::path OidcTokenPath = GetEnvVariable(GetOidcTokenPathEnvVariableName());
+		std::filesystem::path OidcTokenPath = GetEnvVariable(GetOidcTokenPathEnvVariableName()).value_or("");
 		if (IsFile(OidcTokenPath))
 		{
 			return OidcTokenPath;