ngclient/updater: type certainty in download_target()

Re-organise download_target()'s assignment of the target base URL to
ensure mypy can confidently determine that the assigned variable will
never be None.

Signed-off-by: Joshua Lock <jlock@vmware.com>

tuf/ngclient/updater.py

@@ -175,19 +175,19 @@ def download_target(
  TODO: download-related errors
  TODO: file write errors
  """
- if target_base_url is None and self._target_base_url is None:
+ tgt_base_url: str = self._target_base_url or ""
+ if target_base_url is not None:
+ tgt_base_url = _ensure_trailing_slash(target_base_url)
+
+ if not tgt_base_url:
  raise ValueError(
  "target_base_url must be set in either download_target() or "
  "constructor"
  )
- if target_base_url is None:
- target_base_url = self._target_base_url
- else:
- target_base_url = _ensure_trailing_slash(target_base_url)
 
  target_filepath: str = targetinfo["filepath"]
  target_fileinfo: TargetFile = targetinfo["fileinfo"]
- full_url = parse.urljoin(target_base_url, target_filepath)
+ full_url = parse.urljoin(tgt_base_url, target_filepath)
 
  with download.download_file(
  full_url, target_fileinfo.length, self._fetcher