[OptionsResolver] Fix force prepend normalizer

Author: hason

OptionsResolver.php

@@ -532,6 +532,7 @@ public function addNormalizer(string $option, \Closure $normalizer, bool $forceP
         }
 
         if ($forcePrepend) {
+            $this->normalizers[$option] = $this->normalizers[$option] ?? [];
             array_unshift($this->normalizers[$option], $normalizer);
         } else {
             $this->normalizers[$option][] = $normalizer;

Tests/OptionsResolverTest.php

@@ -1506,6 +1506,17 @@ public function testForcePrependNormalizerClosure()
         $this->assertEquals(['foo' => '2nd-normalized-1st-normalized-bar'], $this->resolver->resolve());
     }
 
+    public function testForcePrependNormalizerForResolverWithoutPreviousNormalizers()
+    {
+        // defined by superclass
+        $this->resolver->setDefault('foo', 'bar');
+        $this->resolver->addNormalizer('foo', function (Options $options, $value) {
+            return '1st-normalized-'.$value;
+        }, true);
+
+        $this->assertEquals(['foo' => '1st-normalized-bar'], $this->resolver->resolve());
+    }
+
     public function testAddNormalizerFailsIfUnknownOption()
     {
         $this->expectException('Symfony\Component\OptionsResolver\Exception\UndefinedOptionsException');