qt: allow opening BalanceDialog if warning

Allow to open the BalanceDialog, by clicking on the Pie Diagram in the
bottom left corner of the main window, if there is an active warning,
even if the wallets balance is 0. Right now the user can see the
warning icon for the lighting channel reserve, but cannot click on the
icon to read the actual warning if there is no balance in the wallet.

electrum/gui/qt/balance_dialog.py

@@ -105,6 +105,10 @@ class BalanceToolButton(QToolButton, PieChartObject):
         self._update_size()
         self._warning = False
 
+    @property
+    def has_warning(self) -> bool:
+        return bool(self._warning)
+
     def update_list(self, l, warning: bool):
         self._warning = warning
         self._list = l

electrum/gui/qt/main_window.py

@@ -1804,7 +1804,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
 
     def show_balance_dialog(self):
         balance = self.wallet.get_balances_for_piechart().total()
-        if balance == 0:
+        if balance == 0 and not self.balance_label.has_warning:
             return
         from .balance_dialog import BalanceDialog
         d = BalanceDialog(self, wallet=self.wallet)