tests/common/test_identifiers: Prevent flaky test

There is no guarantee that the directory_with_subdirs strategy will return
a directory with subdirs containing files.

swh/web/tests/common/test_identifiers.py

@@ -156,25 +156,30 @@ def test_get_swhids_info_directory_context(archive_data, directory):
 
     dir_subdir_content = archive_data.directory_ls(dir_subdir["target"])
     dir_subdir_files = [e for e in dir_subdir_content if e["type"] == "file"]
-    dir_subdir_file = random.choice(dir_subdir_files)
+
+    swh_objects_info = [
+        SWHObjectInfo(object_type=DIRECTORY, object_id=dir_subdir["target"])
+    ]
 
     extra_context = {
         "root_directory": directory,
         "path": dir_subdir_path,
-        "filename": dir_subdir_file["name"],
     }
-    swhids = get_swhids_info(
-        [
-            SWHObjectInfo(object_type=DIRECTORY, object_id=dir_subdir["target"]),
+
+    if dir_subdir_files:
+        dir_subdir_file = random.choice(dir_subdir_files)
+        extra_context["filename"] = dir_subdir_file["name"]
+        swh_objects_info.append(
             SWHObjectInfo(
                 object_type=CONTENT, object_id=dir_subdir_file["checksums"]["sha1_git"]
-            ),
-        ],
-        snapshot_context=None,
-        extra_context=extra_context,
+            )
+        )
+
+    swhids = get_swhids_info(
+        swh_objects_info, snapshot_context=None, extra_context=extra_context,
     )
+
     swhid_dir_parsed = get_persistent_identifier(swhids[0]["swhid_with_context"])
-    swhid_cnt_parsed = get_persistent_identifier(swhids[1]["swhid_with_context"])
 
     anchor = get_swh_persistent_id(DIRECTORY, directory)
 
@@ -183,10 +188,13 @@ def test_get_swhids_info_directory_context(archive_data, directory):
         "path": dir_subdir_path,
     }
 
-    assert swhid_cnt_parsed.metadata == {
-        "anchor": anchor,
-        "path": f'{dir_subdir_path}{dir_subdir_file["name"]}',
-    }
+    if dir_subdir_files:
+        swhid_cnt_parsed = get_persistent_identifier(swhids[1]["swhid_with_context"])
+
+        assert swhid_cnt_parsed.metadata == {
+            "anchor": anchor,
+            "path": f'{dir_subdir_path}{dir_subdir_file["name"]}',
+        }
 
 
 @given(revision())