Fix incorrect behavior for empty return types, related to #5

Author: lisachenko

src/Traits/ReflectionFunctionLikeTrait.php

@@ -186,6 +186,8 @@ public function getReturnType()
             $returnType = $returnType->toString();
         } elseif (is_string($returnType)) {
             $isBuiltin = true;
+        } else {
+            return null;
         }
 
         return new ReflectionType($returnType, false, $isBuiltin);

tests/ReflectionFunctionTest.php

@@ -126,6 +126,11 @@ public function testGetReturnTypeMethod()
                     $this->assertSame($originalReturnType->allowsNull(), $parsedReturnType->allowsNull());
                     $this->assertSame($originalReturnType->isBuiltin(), $parsedReturnType->isBuiltin());
                     $this->assertSame($originalReturnType->__toString(), $parsedReturnType->__toString());
+                } else {
+                    $this->assertSame(
+                        $originalRefFunction->getReturnType(),
+                        $refFunction->getReturnType()
+                    );
                 }
             }
         }