[FIX] pad: do not crash during record creation

Backport of 0de067cae9
(and 9b8bc5e5a1)

Rev. c5bd509274 attempted to improve the
pad sync mechanism when merging records (tasks), but failed to consider
the case where the pad_url field is not set yet.
This happens at create(), due to the chicken-and-egg problem with the
pad URL depending on the record ID, and therefore set *after* creation.

Ignoring the sync when the URL is not yet set should be enough, as the
URL generation method also takes care of that first sync.

addons/pad/models/pad.py

@@ -117,7 +117,7 @@ class PadCommon(models.AbstractModel):
     def _set_pad_value(self, vals):
         # Update the pad if the `pad_content_field` is modified
         for k, field in self._fields.iteritems():
-            if hasattr(field, 'pad_content_field') and vals.get(field.pad_content_field):
+            if hasattr(field, 'pad_content_field') and vals.get(field.pad_content_field) and self[k]:
                 company = self.env.user.sudo().company_id
                 myPad = EtherpadLiteClient(company.pad_key, company.pad_server + '/api')
                 path = self[k].split('/p/')[1]