fix import error if note has no "updated" element

[mkorosec]

I got the following error when executing evernote-to-sqlite enex evernote.db evernote.enex

... 
  File "evernote_to_sqlite/cli.py", line 31, in enex
    save_note(db, note)
  File "evernote_to_sqlite/utils.py", line 28, in save_note
    updated = note.find("updated").text
AttributeError: 'NoneType' object has no attribute 'text'

Seems that in some cases the updated element is not added to the note, this is a part of the problematic note:

<created>20201019T074518Z</created>
<note-attributes>
   <source>web.clip7</source>
   <source-application>webclipper.evernote</source-application>
</note-attributes>