cli/command/registry: login: improve flag validation

[thaJeztah]

• relates to Issue with the --username option in docker login when using --password-stdin #5792
• closes Issue with the --username option in docker login when using --password-stdin #5792

cli/command/registry: login: improve flag validation

Before this change, some errors could be ambiguous as they did not
distinguish a flag to be omitted, or set, but with an empty value.

For example, if a user would try to loging but with an empty username,
the error would suggest that the --username flag was not set (which
it was);

I don't have MY_USERNAME set in this shell;

printenv MY_USERNAME || echo 'variable not set'
variable not set

Now, attempting to do a non-interactive login would result in an
ambiguous error;

echo "supersecret" | docker login --password-stdin --username "$MY_USERNAME"
Must provide --username with --password-stdin

With this patch applied, the error indicates that the username was empty,
or not set;

echo "supersecret" | docker login --password-stdin --username "$MY_USERNAME"
username is empty
echo "supersecret" | docker login --password-stdin
the --password-stdin option requires --username to be set
echo "supersecret" | docker login --password-stdin --password "supersecret"
conflicting options: cannot specify both --password and --password-stdin

- Human readable description for the release notes

Improve `docker login` error messages for invalid options.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry/login.go	90.00%	2 Missing ⚠️

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