Fix for crash when cards.xml is missing a <pt> tag

Found while testing with a recent cards.xml. Some cards were missing the <pt> node, set P/T to (0,0) when that's the case.

Example of the error:

looking at  Cardboard Carapace
Traceback (most recent call last):
  File "website.py", line 50, in <module>
    for n in root.findall("./cards/card")))
  File "website.py", line 50, in <genexpr>
    for n in root.findall("./cards/card")))
  File "/home/h/build/mtg-card-scan/search_card.py", line 285, in from_xml_node
    p_t = node.find('pt').text.split('/')
AttributeError: 'NoneType' object has no attribute 'text'