client: fix basic auth with empty password

[dtrifiro]

When authenticating using basic auth and only providing username (no password) the default value of None was cast to string, resulting in an attempt to authenticate using <username>:None instead of providing an empty password.
This broke authentication when providing an auth token as username (https://token@github.com/...)

Also see iterative/dvc#7898 (comment)

[codecov-commenter]

Codecov Report

Merging #980 (f37af59) into master (31490d6) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #980      +/-   ##
==========================================
- Coverage   85.13%   85.05%   -0.08%     
==========================================
  Files          93       93              
  Lines       22959    22980      +21     
  Branches     3368     3368              
==========================================
+ Hits        19545    19546       +1     
- Misses       2953     2983      +30     
+ Partials      461      451      -10     

Impacted Files	Coverage Δ
dulwich/client.py	78.33% <100.00%> (ø)
dulwich/tests/test_client.py	98.27% <100.00%> (+0.04%)	⬆️
dulwich/tests/test_hooks.py	96.42% <0.00%> (-3.58%)	⬇️
dulwich/patch.py	92.39% <0.00%> (-1.64%)	⬇️
dulwich/tests/utils.py	98.60% <0.00%> (-1.40%)	⬇️
dulwich/tests/test_patch.py	95.93% <0.00%> (-1.17%)	⬇️
dulwich/tests/compat/test_client.py	91.22% <0.00%> (-0.93%)	⬇️
dulwich/tests/test_index.py	98.07% <0.00%> (-0.73%)	⬇️
dulwich/repo.py	89.80% <0.00%> (-0.27%)	⬇️
dulwich/config.py	82.07% <0.00%> (-0.26%)	⬇️
... and 1 more

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