markdown_extract causes exception in certain cases

[thriuin]

I recently encountered a case where the markdown_extract function was failing because of the empty value that was being passed in the notes field (we customizing heavily with scheming). The problem could be traced to this line:

if (text is None) or (text.strip() == ''):

This can be avoided by simplifying this test to:

if not text:

PR to follow.

[wardi]

fixed via 305de4d