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, 3 insertions, 3 deletions

diff --git a/src/zenutil/invocationhistory.cpp b/src/zenutil/invocationhistory.cpp
index 077061752..a022e4cd5 100644
--- a/src/zenutil/invocationhistory.cpp
+++ b/src/zenutil/invocationhistory.cpp
@@ -51,10 +51,10 @@ namespace {
 	std::filesystem::path ResolveHistoryDir()
 	{
 #if ZEN_PLATFORM_WINDOWS
-		std::string LocalAppData = GetEnvVariable("LOCALAPPDATA");
-		if (!LocalAppData.empty())
+		std::optional<std::string> LocalAppData = GetEnvVariable("LOCALAPPDATA");
+		if (LocalAppData && !LocalAppData->empty())
 		{
-			return std::filesystem::path(LocalAppData) / "Epic" / "Zen" / "History";
+			return std::filesystem::path(*LocalAppData) / "Epic" / "Zen" / "History";
 		}
 #endif
 		std::filesystem::path SystemRoot = PickDefaultSystemRootDirectory();