Encapsed strings should return non-falsy-string if non-falsy like concat

[kkmuffme]

https://psalm.dev/r/182c160a4e

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/182c160a4e

<?php

/**
 * @var string $a
 */

$b = "hello {$a} world";
/** @psalm-trace $b */;

$c = "hello" .  $a . "world";
/** @psalm-trace $c */;

Psalm output (using commit 08afc45):

INFO: Trace - 8:23 - $b: non-empty-string

INFO: Trace - 11:23 - $c: non-falsy-string

INFO: UnusedVariable - 7:1 - $b is never referenced or the value is not used

INFO: UnusedVariable - 10:1 - $c is never referenced or the value is not used