kivy: fix history tab fiat values

follow-up 46f1fca7c3

electrum/gui/kivy/uix/screens.py

@@ -152,7 +152,7 @@ class HistoryScreen(CScreen):
                 fx = self.app.fx
                 fiat_value = value / Decimal(bitcoin.COIN) * self.app.wallet.price_at_timestamp(tx_hash, fx.timestamp_rate)
                 fiat_value = Fiat(fiat_value, fx.ccy)
-                ri['quote_text'] = str(fiat_value)
+                ri['quote_text'] = fiat_value.to_ui_string()
         return ri
 
     def update(self, see_all=False):

electrum/util.py

@@ -178,6 +178,12 @@ class Fiat(object):
         else:
             return "{:.2f}".format(self.value)
 
+    def to_ui_string(self):
+        if self.value is None or self.value.is_nan():
+            return _('No Data')
+        else:
+            return "{:.2f}".format(self.value) + ' ' + self.ccy
+
     def __eq__(self, other):
         if self.ccy != other.ccy:
             return False