fix NullableType and UnionType

Author: duhon

composer.json

@@ -15,7 +15,7 @@
         "phpstan/phpdoc-parser": "^0.5.5",
         "sabre/xml": "~2.2.3",
         "symfony/console": "~4.4.0||~5.4.0",
-        "tomzx/php-semver-checker": "^0.15.0",
+        "tomzx/php-semver-checker": "^0.16.0",
         "wikimedia/less.php": "^3.2"
     },
     "require-dev": {

src/Analyzer/ClassMethodAnalyzer.php

@@ -459,7 +459,11 @@ private function getDocReturnDeclaration(ClassMethod $method)
             ($parsedComment = $method->getAttribute('docCommentParsed'))
             && isset($parsedComment['return'])
         ) {
-            $result = implode('|', $parsedComment['return']);
+            if ($parsedComment['return'][0] instanceof NullableType) {
+                $result =  '?'.$parsedComment['return'][0]->type;
+            } else {
+                $result = implode('|', $parsedComment['return']);
+            }
 
             return $result;
         } elseif ($this->dependencyGraph !== null) {