Password complexity check should be skipped on external auth sources

[silverwind]

• Gitea version (or commit ref): ee52498

Description

When adding a new user using a external auth source like LDAP, the option PASSWORD_COMPLEXITY is validated which prevents a admin from creating a new user with below error message while there is no actual password field visible in the UI. This check should be skipped for cases where gitea does not actually store a password.

Screenshots

[guillep2k]

Looks like the root cause for #8558