Check if date exists before attempting to parse it

[Plsr]

If one of the Webmonday Events does not have a date field, the pares_date function will throw an error:

TypeError at /
no implicit conversion of nil into String

Now, this issue at hand has its origin in the merging of the webmondays.yml files, where one of the webmondays did not have a date field (see 49c32b2). However, we should make sure this won't happen again in the future by checking if the date field exists, shouldn't we?