Fix constant initialiser

[marksteward]

This should be data=None with a check that initialises it:

https://github.com/pubstandards/pubstandards-london/blob/master/ps_data.py#L22

(it doesn't currently matter)

[jaylett]

Top re-entrancy :-)

(The reason it doesn't currently matter is so subtle that it makes me want to rewrite the PSEvent constructor to make things…more clear.)