Don't filter out empty environment variables

This check was only needed because `operator[]` inserts an empty entry
if the variable doesn't exist. Wine also complains when `WINEPREFIX` is
empty, so we should probably not try to have our own behavior here.

src/common/utils.cpp

@@ -42,10 +42,10 @@ constexpr char temp_dir_override_env_var[] = "YABRIDGE_TEMP_DIR";
 fs::path get_temporary_directory() {
     const bp::environment env = boost::this_process::environment();
     if (const auto directory = env.find(temp_dir_override_env_var);
-        directory != env.end() && !directory->empty()) {
+        directory != env.end()) {
         return directory->to_string();
     } else if (const auto directory = env.find("XDG_RUNTIME_DIR");
-               directory != env.end() && !directory->empty()) {
+               directory != env.end()) {
         return directory->to_string();
     } else {
         return fs::temp_directory_path();

src/plugin/utils.cpp

@@ -257,8 +257,7 @@ fs::path normalize_plugin_path(const fs::path& windows_library_path,
 std::variant<OverridenWinePrefix, fs::path, DefaultWinePrefix> find_wine_prefix(
     fs::path windows_plugin_path) {
     const bp::environment env = boost::this_process::environment();
-    if (const auto prefix = env.find("WINEPREFIX");
-        prefix != env.end() && !prefix->empty()) {
+    if (const auto prefix = env.find("WINEPREFIX"); prefix != env.end()) {
         return OverridenWinePrefix{prefix->to_string()};
     }