fix(qt): an EMPTY Qt backend env var must not disable the renderer preference

apply_renderer_preference gated on var_os("QSG_RHI_BACKEND").is_some(), which
answers YES for a variable that is present with no value. Clearing a variable
rather than unsetting it is the common way to end up there - `QSG_RHI_BACKEND=
./qbz` in a shell, and on Windows PowerShell's SetEnvironmentVariable(name,
$null), which leaves the name present and empty. The whole preference then
stopped applying, the user's persisted Settings choice included, and the only
clue was one info line claiming an explicit backend was present.

Found on the Windows box, where a test harness cleared the variable that way
and the renderer arm silently never ran. It reproduces on Linux the same way;
this is not a platform fix.

QBZ_RENDERER, read a few lines below, has always filtered the empty string.
This applies the same rule to the two Qt variables. The predicate is split
from the lookup so the three new tests exercise it without mutating
process-global env, which would race the other tests in the binary.

cc

crates/qbz-qt/src/main.rs

@@ -3034,6 +3034,56 @@ fn publish_home_sections(sections: &home_qt::DiscoverSections) {
     });
 }
 
+/// Does an explicit Qt backend environment variable actually override the
+/// renderer preference?
+///
+/// `var_os(..).is_some()` answers YES for an EMPTY value, and an empty value
+/// is exactly what a shell or a test harness leaves behind when it clears a
+/// variable rather than unsetting it: `QSG_RHI_BACKEND= ./qbz` on Linux, and
+/// on Windows PowerShell's `SetEnvironmentVariable(name, $null)`, which leaves
+/// the name present with no value. The whole preference then silently stopped
+/// applying, the persisted Settings choice included, and the only clue was one
+/// info line claiming an explicit backend was present.
+///
+/// `QBZ_RENDERER`, read a few lines below, has always filtered the empty
+/// string. This is the same rule for the two Qt variables.
+fn qt_backend_env_overrides(name: &str) -> bool {
+    env_value_overrides(std::env::var_os(name))
+}
+
+/// The predicate above, split from the lookup so it can be tested without
+/// mutating process-global state, which races other tests in the same binary.
+fn env_value_overrides(value: Option<std::ffi::OsString>) -> bool {
+    value
+        .map(|v| !v.to_string_lossy().trim().is_empty())
+        .unwrap_or(false)
+}
+
+#[cfg(test)]
+mod renderer_env_tests {
+    use super::env_value_overrides;
+    use std::ffi::OsString;
+
+    #[test]
+    fn an_absent_variable_does_not_override() {
+        assert!(!env_value_overrides(None));
+    }
+
+    #[test]
+    fn an_empty_variable_does_not_override() {
+        // The defect: `QSG_RHI_BACKEND=` disabled the entire renderer
+        // preference, the user's persisted setting included.
+        assert!(!env_value_overrides(Some(OsString::from(""))));
+        assert!(!env_value_overrides(Some(OsString::from("   "))));
+    }
+
+    #[test]
+    fn a_real_value_still_overrides() {
+        assert!(env_value_overrides(Some(OsString::from("opengl"))));
+        assert!(env_value_overrides(Some(OsString::from(" d3d11 "))));
+    }
+}
+
 /// The Settings > Appearance RENDERER row, consumed at startup (PARITY-DEBT
 /// #104 — the row rendered and persisted into the SAME ui_prefs the Slint
 /// reads, but nothing consumed it, so it lied).
@@ -3058,8 +3108,7 @@ fn apply_renderer_preference() {
     // backend must be undone even when this one is being overridden by env.
     let reverted = renderer_qt::revert_if_previous_launch_died();
 
-    if std::env::var_os("QSG_RHI_BACKEND").is_some()
-        || std::env::var_os("QT_QUICK_BACKEND").is_some()
+    if qt_backend_env_overrides("QSG_RHI_BACKEND") || qt_backend_env_overrides("QT_QUICK_BACKEND")
     {
         log::info!("[renderer] explicit Qt backend env present; leaving the choice to it");
         return;