Do not print caret when offset is None.

Lib/traceback.py

@@ -175,6 +175,7 @@ def format_exception_only(etype, value):
                 while i < len(line) and line[i].isspace():
                     i = i+1
                 list.append('    %s\n' % line.strip())
+                if offset is not None:
                     s = '    '
                     for c in line[i:offset-1]:
                         if c.isspace():