[FIX] base: don't fail on empty groups attribute

In 15.0 this was supported. It may be also handy when editing views to
momentarily set the groups to `""`.

Steps to reproduce:
1. Install Odoo 15 locally
2. Edit or create a view with `groups=""` for some component
3. Upgrade to 16.
It fails.

Empty groups was allowed in 15.0 we want to ensure this is not broken
unintentionally anymore, such a new test was added.

Muted logged to hide the warning (also present in 15.0):
```
2023-02-08 11:09:18,697 506777 WARNING test_16_gr odoo.addons.base.models.ir_ui_view: The group '' defined in view does not exist!
View error context:
{'file': None,
 'line': 3,
 'name': 'foo',
 'view': ir.ui.view(242,),
 'view.model': 'res.partner',
 'view.parent': ir.ui.view(),
 'xmlid': ''}
 ```

closes odoo/odoo#112246

X-original-commit: 4379dce95edcc34a8e97e73a3bdaa2e20fdc79a8
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>

odoo/addons/base/models/ir_ui_view.py

@@ -1061,7 +1061,8 @@ actual arch.
         """
 
         for node in tree.xpath('//*[@groups]'):
-            if not self.user_has_groups(node.attrib.pop('groups')):
+            attrib_groups = node.attrib.pop('groups')
+            if attrib_groups and not self.user_has_groups(attrib_groups):
                 node.getparent().remove(node)
             elif node.tag == 't' and (not node.attrib or node.get('postprocess_added')):
                 # Move content of <t groups=""> blocks

odoo/addons/base/tests/test_views.py

@@ -2969,6 +2969,23 @@ class TestViews(ViewCase):
             </form>
         """, valid=True)
 
+    @mute_logger('odoo.addons.base.models.ir_ui_view')
+    def test_empty_groups_attrib(self):
+        """Ensure we allow empty groups attribute"""
+        view = self.View.create({
+            'name': 'foo',
+            'model': 'res.partner',
+            'arch': """
+                <form>
+                    <field name="name" groups="" />
+                </form>
+            """,
+        })
+        arch = self.env['res.partner'].get_view(view_id=view.id)['arch']
+        tree = etree.fromstring(arch)
+        nodes = tree.xpath("//field[@name='name' and not (@groups)]")
+        self.assertEqual(1, len(nodes))
+
     def test_attrs_groups_with_groups_in_model(self):
         """Tests the attrs is well processed to modifiers for a field node combining:
         - a `groups` attribute on the field node in the view architecture