resolve merge directives in new nested config subtrees

Author: michalsn

system/Config/BaseConfig.php

@@ -435,8 +435,11 @@ private function mergeByKey(array $current, array $incoming): array
                 }
             } elseif (is_int($key)) {
                 $current[] = $value;
-            } elseif (isset($current[$key]) && is_array($current[$key]) && is_array($value)) {
-                $current[$key] = $this->mergeByKey($current[$key], $value);
+            } elseif (is_array($value)) {
+                $current[$key] = $this->mergeByKey(
+                    isset($current[$key]) && is_array($current[$key]) ? $current[$key] : [],
+                    $value,
+                );
             } else {
                 $current[$key] = $value;
             }

tests/system/Config/MergeTest.php

@@ -340,6 +340,17 @@ public function testByKeyResolvesBrandNewNestedArraySubtree(): void
         $this->assertSame(['deep' => ['nested' => ['value']]], $result);
     }
 
+    public function testByKeyResolvesDirectiveInsideBrandNewNestedArraySubtree(): void
+    {
+        $result = $this->apply([], Merge::byKey([
+            'globals' => [
+                'before' => Merge::append(['auth']),
+            ],
+        ]));
+
+        $this->assertSame(['globals' => ['before' => ['auth']]], $result);
+    }
+
     public function testAppendPayloadIsTerminalLiteral(): void
     {
         // A directive embedded in an append() payload is literal data, not interpreted.