Fix crash in console output when colors are disabled

Without any colors, TopydoString doesn't have any colors set resulting
in an empty markup item. That made the urwid library trip.

topydo/ui/ConsoleWidget.py

@@ -34,6 +34,12 @@ def topydostringToMarkup(p_string):
     markup = []
 
     color_positions = sorted(p_string.colors.items())
+
+    # in case no color positions are available, at least set something at the
+    # start position
+    if not color_positions:
+        color_positions = [(0, None)]
+
     for i, (start_pos, color) in enumerate(color_positions):
         # color starts at indicated position
         start = start_pos