[FIX] Install l10n_eu_oss without chart template

When installing l10n_eu_oss without a localization installed, a line fails
in the get_oss_tags function of res_company, thus halting the installation
and returning an error.

This fail is due to the function relying on the company in self having its
'chart_template_id' set, when this is not always the case.

Adding a failsafe to that function allows the installation of the module to
proceed in the event that the chart_template_id is not set.

opw-3291118
opw-3289913

closes odoo/odoo#120104

X-original-commit: 4b140806
Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>
Signed-off-by: Huylenbroeck Florent (flhu) <flhu@odoo.com>

addons/l10n_eu_oss/models/res_company.py

@@ -127,7 +127,9 @@ class Company(models.Model):
 
     def _get_oss_tags(self):
         oss_tag = self.env.ref('l10n_eu_oss.tag_oss')
-        [chart_template_xml_id] = self.chart_template_id.parent_id.get_xml_id().values() or self.chart_template_id.get_xml_id().values()
+        chart_template_xml_id = ''
+        if self.chart_template_id:
+            [chart_template_xml_id] = self.chart_template_id.parent_id.get_external_id().values() or self.chart_template_id.get_external_id().values()
         tag_for_country = EU_TAG_MAP.get(chart_template_xml_id, {
             'invoice_base_tag': None,
             'invoice_tax_tag': None,