fix empty string not quoted in keyed array offset

[orklah]

This will fix an error I thought about while looking for another bug.

The error can be seen here but it applies more widely, for example on psalter fixes for MissingReturnType with the same structure

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/3e6d50f022

<?php

class A
{
    function test(): void
    {
        $a = ['' => ''];

        /** @var array<string, mixed> $b */
        $b = [];

        $this->assertSame($a, $b);
    }

    /**
     * @template T
     * @param T      $expected
     * @psalm-assert =T $actual
     */
    public function assertSame(mixed $expected, mixed $actual): void
    {
        return;
    }
}

Psalm output (using commit b8240e2):

ERROR: InvalidDocblock - 12:16 - array{: ""} cannot be used in an assertion