Show items without priority in console

When topydo is run with colors, items without priority get an invalid
markup added. Only add markup when an item has actually a priority.

Bug found and fixed by @mruwek

topydo/ui/columns/ConsoleWidget.py

@@ -66,7 +66,9 @@ def topydostringToMarkup(p_string):
     # markup
     if color_at_start and isinstance(p_string.metadata, Todo):
         priority = p_string.metadata.priority()
-        markup = ('pri_' + priority, markup)
+
+        if priority:
+            markup = ('pri_' + priority, markup)
 
     return markup