wxGUI/dbmgr: fix get statistics for character column type if it contains NULL value (#1574)

gui/wxpython/dbmgr/base.py

@@ -3932,7 +3932,8 @@ class FieldStatistics(wx.Frame):
                      message=_("Unable to calculte standard deviation."))
         varSum = 0
         for var in decode(dataVar).splitlines():
-            varSum += float(var)
+            if var:
+                varSum += float(var)
         stddev = math.sqrt(varSum / count)
 
         self.SetTitle(_("Field statistics <%s>") % column)