fix: fall back to userPrincipalName when mail is empty in AzureADv2

[Yanhu007]

Fixes #289

Problem

The AzureADv2 provider maps user.Email from the Microsoft Graph API mail field. However, many Azure AD accounts (especially personal Microsoft accounts and some organizational accounts) have mail as null/empty while userPrincipalName contains their email address.

This results in an empty Email field on the returned user.

Fix

Fall back to userPrincipalName when mail is empty:

user.Email = u.Email
if user.Email == "" {
    user.Email = u.UserPrincipalName
}

This matches the approach used by other OAuth providers (e.g., Google) that check multiple fields for email.