HasmMap: fixed empty property name access

Introduced by 3891625cd1

PostgreSQL uses '::' syntax for type casting. Before this fix, HashMap returned ':\xff:' during SQL translation/substitution.

src/Dibi/HashMap.php

@@ -58,7 +58,7 @@ final class HashMap extends HashMapBase
 	{
 		if ($nm == '') {
 			$nm = "\xFF";
-			return $this->$nm = $this->$nm ?? $this->getCallback()('');
+			return isset($this->$nm) ? $this->$nm : $this->$nm = $this->getCallback()('');
 		} else {
 			return $this->$nm = $this->getCallback()($nm);
 		}

tests/dibi/HashMap.phpt

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+use Tester\Assert;
+
+require __DIR__ . '/bootstrap.php';
+
+$hash = new Dibi\HashMap(function ($v) {
+	return "b-$v-e";
+});
+
+Assert::same('b-X-e', $hash->{'X'});
+Assert::same('b--e', $hash->{''});