qt send tab: "max" btn should not raise NotEnoughFunds due to fees

If a tx cannot be constructed due to current fee settings, try to
create one with zero miner fees instead and let user to change the
fee later.

fixes #6755

electrum/gui/qt/main_window.py

@@ -1428,8 +1428,13 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             is_sweep=False)
 
         try:
-            tx = make_tx(None)
-        except (NotEnoughFunds, NoDynamicFeeEstimates, MultipleSpendMaxTxOutputs) as e:
+            try:
+                tx = make_tx(None)
+            except (NotEnoughFunds, NoDynamicFeeEstimates) as e:
+                # Check if we had enough funds excluding fees,
+                # if so, still provide opportunity to set lower fees.
+                tx = make_tx(0)
+        except (MultipleSpendMaxTxOutputs, NotEnoughFunds) as e:
             self.max_button.setChecked(False)
             self.show_error(str(e))
             return