fix(django): avoid empty span name on empty path

when using `path('',...)`, previous code produces spans with an empty
string name. This change corrects this fallback behavior and adds a
test.

instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/middleware/otel_middleware.py

@@ -172,7 +172,7 @@ def _get_span_name(request):
             else:
                 match = resolve(request.path)
 
-            if hasattr(match, "route"):
+            if hasattr(match, "route") and match.route:
                 return match.route
 
             # Instead of using `view_name`, better to use `_func_name` as some applications can use similar

instrumentation/opentelemetry-instrumentation-django/tests/test_middleware.py

@@ -75,6 +75,7 @@
 
 if DJANGO_2_0:
     from django.urls import re_path
+    from django.urls import path
 else:
     from django.conf.urls import url as re_path
 
@@ -87,6 +88,7 @@
     re_path(r"^excluded_noarg/", excluded_noarg),
     re_path(r"^excluded_noarg2/", excluded_noarg2),
     re_path(r"^span_name/([0-9]{4})/$", route_span_name),
+    path("", traced, name="empty"),
 ]
 _django_instrumentor = DjangoInstrumentor()
 
@@ -207,6 +209,18 @@ def test_not_recording(self):
             self.assertFalse(mock_span.set_attribute.called)
             self.assertFalse(mock_span.set_status.called)
 
+    def test_empty_path(self):
+        Client().get("/")
+
+        spans = self.memory_exporter.get_finished_spans()
+        self.assertEqual(len(spans), 1)
+
+        span = spans[0]
+
+        self.assertEqual(
+            span.name, "empty"
+        )
+
     def test_traced_post(self):
         Client().post("/traced/")