Fix DocBlockTypeResolver crash on PHP 7.3 and less

tests/Fixtures/ObjectWithPhpDocProperty.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JMS\Serializer\Tests\Fixtures;
+
+// phpcs:disable
+final class ObjectWithPhpDocProperty
+{
+    /**
+     */
+    private $emptyBlock;
+
+}

tests/Metadata/Driver/DocBlockTypeResolverTest.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace JMS\Serializer\Tests\Metadata\Driver;
+
+use JMS\Serializer\Metadata\Driver\DocBlockTypeResolver;
+use JMS\Serializer\Tests\Fixtures\ObjectWithPhpDocProperty;
+use PHPUnit\Framework\TestCase;
+use ReflectionProperty;
+
+final class DocBlockTypeResolverTest extends TestCase
+{
+    public function testGetPropertyDocblockTypeHintDoesNotCrash(): void
+    {
+        // It crashed on PHP 7.3 and less because of array_merge(...[])
+
+        $resolver = new DocBlockTypeResolver();
+        self::assertNull(
+            $resolver->getPropertyDocblockTypeHint(
+                new ReflectionProperty(ObjectWithPhpDocProperty::class, 'emptyBlock')
+            )
+        );
+    }
+}