Fix an unexpected exception when using dsn without username & password

[willww64]

Description

When the -u, --user flag is not provided and a DSN without the user:pass@ part is used (such as mysql://localhost:3306/mydb), an unexpected exception occurs. The expected behavior should be fallback to the current logged-in user (line 420 of main.py, in the connect method).

This is caused by an unchecked None value for uri.username. This pull request fixes this bug.

Example exception output:

$ mycli mysql://localhost:3306/mydb
Traceback (most recent call last):
  File "/usr/bin/mycli", line 33, in <module>
    sys.exit(load_entry_point('mycli==1.27.0', 'console_scripts', 'mycli')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/mycli/main.py", line 1282, in cli
    user = unquote(uri.username)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/parse.py", line 671, in unquote
    if '%' not in string:
       ^^^^^^^^^^^^^^^^^
TypeError: argument of type 'NoneType' is not iterable

Checklist

• I've added this contribution to the changelog.md.
• I've added my name to the AUTHORS file (or it's already there).