Merge #534: Fix sweep in GUI

a51bd55903 Fix sweep in GUI (Kristaps Kaupe)

Pull request description:

  Same fix as #533, but for GUI. Fixes bug introduced with #529.

Top commit has no ACKs.

Tree-SHA512: 4a347307ce7b306087c202220b8f68f14022362051fb5e02f4c59a51aed1786092dde0ea82abde4f6eaa4461a7296ce7967803ef0a121075e87f9965e4bcbf42

scripts/joinmarket-qt.py

@@ -631,7 +631,8 @@ class SpendTab(QWidget):
         btc_amount_str = self.widgets[3][1].text()
         # for coinjoin sends no point to send below dust threshold, likely
         # there will be no makers for such amount.
-        if makercount != 0 and not checkAmount(self, btc_amount_str):
+        if (makercount != 0 and btc_amount_str != '0' and
+            not checkAmount(self, btc_amount_str)):
             return
         amount = btc.amount_to_sat(btc_amount_str)
         if makercount == 0: