[FIX] account: P3 invalid comparison

Python 3 removed "universal"
ordering. ``ResCompany.fiscalyear_lock_date`` is not required and thus
needs a fallback in case it's empty lest we try comparing a string and
None or False, which is an error.

addons/account/models/account_move.py

@@ -1847,6 +1847,6 @@ class AccountFullReconcile(models.Model):
         # The move date should be the maximum date between payment and invoice
         # (in case of payment in advance). However, we should make sure the
         # move date is not recorded after the end of year closing.
-        if move_date > company.fiscalyear_lock_date:
+        if move_date > (company.fiscalyear_lock_date or '0000-00-00'):
             res['date'] = move_date
         return res