Allow saving the empty default notify target

An empty target means notifications are not configured yet and is the
shipped default; only malformed strings (padded or whitespace-only)
reject.

custom_components/nestquest/options_flow.py

@@ -98,11 +98,15 @@ def _async_validate(user_input: dict[str, Any]) -> dict[str, str]:
         ):
             errors[time_key] = "invalid_time"
     notify_target = user_input.get(CONF_NOTIFY_TARGET)
-    if notify_target is not None and (
-        not isinstance(notify_target, str)
-        or not notify_target.strip()
-        or notify_target != notify_target.strip()
+    if notify_target is not None and not isinstance(notify_target, str):
+        errors[CONF_NOTIFY_TARGET] = "invalid"
+    elif isinstance(notify_target, str) and notify_target != (
+        notify_target.strip()
     ):
+        # An EMPTY target is valid — it means "no notifications
+        # configured yet" and is the shipped default; a value that
+        # differs from its own trim (padded or whitespace-only) is
+        # malformed and rejected.
         errors[CONF_NOTIFY_TARGET] = "invalid"
     for toggle_key in (
         CONF_MORNING_SUMMARY_ENABLED,

tests/test_options_flow.py

@@ -712,9 +712,22 @@ def test_options_flow_notification_times_must_be_strict_hh_mm() -> None:
         assert result["errors"] == {CONF_MORNING_SUMMARY_TIME: "invalid_time"}
 
 
-def test_options_flow_notify_target_must_be_clean_nonempty_string() -> None:
+def test_options_flow_notify_target_validation() -> None:
+    """The empty target is VALID (it is the shipped default and must be
+    savable); only malformed non-empty values reject."""
     flow = _make_flow(_make_entry())
-    for bad_target in ("", "  notify.x  ", 42):
+    # The empty default saves cleanly.
+    result = _run(
+        flow.async_step_init(
+            {**VALID_INPUT, CONF_NOTIFY_TARGET: ""}
+        )
+    )
+    assert result["type"] == "create_entry"
+    assert result["data"][CONF_NOTIFY_TARGET] == ""
+
+    # Malformed non-empty values reject.
+    for bad_target in ("  notify.x  ", "   ", 42):
+        flow = _make_flow(_make_entry())
         result = _run(
             flow.async_step_init(
                 {