Add validation to prevent empty username/email in UserController update

[Copilot]

The UserController::update() method allowed empty strings from POST to overwrite valid credentials because $_POST['username'] ?? $user->getUsername() treats empty string as a valid value.

Changes

• Added null checks to distinguish between omitted fields and empty submissions
• Trim and cast input values before validation
• Reject updates with empty username or email, redirect with error message
• Preserve existing user data when fields are not in POST payload

// Before
$username = $_POST['username'] ?? $user->getUsername();
$email = $_POST['email'] ?? $user->getEmail();

// After
$usernameInput = $_POST['username'] ?? null;
$emailInput = $_POST['email'] ?? null;

$username = $usernameInput !== null ? trim( (string)$usernameInput ) : $user->getUsername();
$email = $emailInput !== null ? trim( (string)$emailInput ) : $user->getEmail();

if( $username === '' || $email === '' )
{
    $this->redirect( 'admin_users_edit', ['id' => $id], ['error', 'Username and email are required'] );
}

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