[DeadCode] Skip RemoveAlwaysTrueIfConditionRector on maybe undefined variable

[TomasVotruba]

Fixes rectorphp/rector#9904

A variable only assigned inside conditional branches is maybe-undefined. On code paths where it stays undefined, PHP treats it as null, so the if condition is not always true. The rule now skips removal when a condition variable is not certainly defined in the current scope.

Before:

if ($username !== INTERFACE_USERNAME) {
    $error = true;
}
if ($password !== INTERFACE_PASSWORD) {
    $error = true;
}
if ($error) { // wrongly removed, $error can be undefined
    $response->setAttribute('status', 'ERROR');
}