[FIX] im_livechat: prevent crash in rule matching if URL not set

The URL might not be set because it comes from `referer`, in that case match the
first rule with no `regex_url`.

It crashes before this commit because `re.search()` is expecting a string and
`headers.get()` is defaulting to `None`.

opw-2205202

closes odoo/odoo#46593

X-original-commit: e2f8caf07b2f749935df9184ded738cc1c482423
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/im_livechat/models/im_livechat_channel.py

@@ -270,7 +270,9 @@ class ImLivechatChannelRule(models.Model):
         """
         def _match(rules):
             for rule in rules:
-                if re.search(rule.regex_url or '', url):
+                # url might not be set because it comes from referer, in that
+                # case match the first rule with no regex_url
+                if re.search(rule.regex_url or '', url or ''):
                     return rule
             return False
         # first, search the country specific rules (the first match is returned)