tests/test-backwards-remove.t

url: do not continue HTTP authentication with user=None (issue6425)

I initially thought this is a py3-compat bug of passwordmgr._writedebug(),
but actually returning (None, str) pair is wrong at all. HTTP authentication
would continue with user="None" in that case.

Since registering a password of user=None should also be wrong, this patch
simply adds early return.

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls -A
  .hg
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls -A
  .hg
  a