fail-on-template-vars: improve compatibility with Django behavior

With `OneToOneField`, Django raises `Model.DoesNotExist` which is
converted by its template engine to `string_if_invalid`:
https://github.com/django/django/blob/5.0.7/django/template/base.py#L932-L933

It is usually falsy, hence the need for `InvalidVarException.__bool__`
to return `bool(self.origin_value)` to be consistent with Django's
default behavior.

However to trigger `InvalidVarException` behavior and its dreaded
`InvalidVarException.__mod__`, it needs to go through this check:
https://github.com/django/django/blob/5.0.7/django/template/base.py#L716
and thus also needs to be truthy hence the stack inspecting `__bool__`
method to know what to return.

Author: xavfernandez

pytest_django/plugin.py

@@ -688,6 +688,13 @@ def _get_origin() -> str | None:
                 return name
             return None
 
+        def __bool__(self) -> bool:
+            for frame_info in inspect.stack():
+                if frame_info.function == "resolve" and frame_info.filename.endswith("base.py"):
+                    # To go through this guard: https://github.com/django/django/blob/5.0.7/django/template/base.py#L716
+                    return True
+            return bool(self.origin_value)
+
         def __mod__(self, var: str) -> str:
             origin = self._get_origin()
             if origin:

tests/test_environment.py

@@ -188,6 +188,50 @@ def test_for_invalid_template(client):
     )
 
 
+@pytest.mark.django_project(
+    extra_settings="""
+    TEMPLATE_LOADERS = (
+        'django.template.loaders.filesystem.Loader',
+        'django.template.loaders.app_directories.Loader',
+    )
+    """
+)
+def test_invalid_template_variable_object_does_not_exists_behaves_like_an_empty_string(
+    django_pytester: DjangoPytester
+) -> None:
+    django_pytester.create_app_file(
+        "<div>{% if object_exists %}This should not appear{% endif %}</div>",
+        "templates/invalid_template_base.html",
+    )
+    django_pytester.create_app_file(
+        "{% include 'invalid_template_base.html' %}", "templates/invalid_template.html"
+    )
+    django_pytester.create_test_module(
+        """
+        from django.core.exceptions import ObjectDoesNotExist
+        from django.template.loader import render_to_string
+
+        import pytest
+
+        def fake_one_to_one_relation_missing():
+            raise ObjectDoesNotExist()
+
+        def test_ignore(client):
+            assert render_to_string(
+                'invalid_template.html',
+                {"object_exists": fake_one_to_one_relation_missing},
+            ) == "<div></div>"
+        """
+    )
+    result = django_pytester.runpytest_subprocess("-s", "--fail-on-template-vars")
+
+    result.stdout.fnmatch_lines_random(
+        [
+            "tpkg/test_the_test.py .",
+        ]
+    )
+
+
 @pytest.mark.django_project(
     extra_settings="""
     TEMPLATE_LOADERS = (