handle undefined position.line_range in case of comments on empty files

[woutdenolf]

Closes #250

Example when the TypeError happens

        {
          "id": "e6b155b372d15d12fcd6b2ced5289313c8b178b5",
          "individual_note": false,
          "notes": [
            {
              "id": 482339,
              "type": "DiffNote",
              "body": "empty file. I guess it should be removed from the PR.",
              "author": {...},
              "created_at": "2025-10-27T10:40:33.686+01:00",
              "updated_at": "2025-10-27T13:24:19.959+01:00",
              "system": false,
              "noteable_id": 38062,
              "noteable_type": "MergeRequest",
              "project_id": 3336,
              "commit_id": null,
              "position": {
                "base_sha": "8017811eb76e7144a91f8e62daff8636f03e14d4",
                "start_sha": "8017811eb76e7144a91f8e62daff8636f03e14d4",
                "head_sha": "39ac4167acb2e01be9a0c4b50f0db2a3bd181939",
                "old_path": "src/ewoksutils/tests/cli_test_utils.py",
                "new_path": "src/ewoksutils/tests/cli_test_utils.py",
                "position_type": "file"
              },
              "resolvable": true,
              "resolved": true,
              "resolved_by": {...},
              "resolved_at": "2025-10-27T13:24:19.959+01:00",
              "suggestions": [],
              "confidential": false,
              "internal": false,
              "imported": false,
              "imported_from": "none",
              "noteable_iid": 58,
              "commands_changes": {}
            },
            {
              "id": 482462,
              "type": "DiffNote",
              "body": "changed this file in [version 7 of the diff](...",
              "author": {...},
              "created_at": "2025-10-27T13:23:57.320+01:00",
              "updated_at": "2025-10-27T13:23:57.320+01:00",
              "system": true,
              "noteable_id": 38062,
              "noteable_type": "MergeRequest",
              "project_id": 3336,
              "commit_id": null,
              "position": {
                "base_sha": "8017811eb76e7144a91f8e62daff8636f03e14d4",
                "start_sha": "8017811eb76e7144a91f8e62daff8636f03e14d4",
                "head_sha": "39ac4167acb2e01be9a0c4b50f0db2a3bd181939",
                "old_path": "src/ewoksutils/tests/cli_test_utils.py",
                "new_path": "src/ewoksutils/tests/cli_test_utils.py",
                "position_type": "file"
              },
              "resolvable": false,
              "suggestions": [],
              "confidential": false,
              "internal": false,
              "imported": false,
              "imported_from": "none",
              "noteable_iid": 58,
              "commands_changes": {}
            }
          ]
        },

This MR fixes the issue in these cases by

• modifying the slug to point to the file in the compare (not a line in the file, because it is an empty file)
• not adding the line-range link when position.line_range is not defined