Set nullable true when default value is null

When reflection shows that the default parameter of a a property is
null, then the `nullable` parameter is set to `true` for that property.

Currently that needs to be set via the attribute but that is a bit
redundant as the information is already available from reflection

This change only does that for properties and for setter methods.

Right now getter methods that allow a null value to be returned are not
taken into account for setting the `nullable` property.

src/ModelDescriber/Annotations/ReflectionReader.php

@@ -55,15 +55,21 @@ public function updateProperty(
             $methodDefault = $this->getDefaultFromMethodReflection($reflection);
             if (Generator::UNDEFINED !== $methodDefault) {
                 $property->default = $methodDefault;
+                if (null === $methodDefault) {
+                    $property->nullable = true;
+                }
 
                 return;
             }
         }
 
         if ($reflection instanceof \ReflectionProperty) {
-            $methodDefault = $this->getDefaultFromPropertyReflection($reflection);
-            if (Generator::UNDEFINED !== $methodDefault) {
-                $property->default = $methodDefault;
+            $propertyDefault = $this->getDefaultFromPropertyReflection($reflection);
+            if (Generator::UNDEFINED !== $propertyDefault) {
+                $property->default = $propertyDefault;
+                if (null === $propertyDefault) {
+                    $property->nullable = true;
+                }
 
                 return;
             }
@@ -90,6 +96,9 @@ public function updateProperty(
             }
 
             $property->default = $parameter->getDefaultValue();
+            if (null === $property->default) {
+                $property->nullable = true;
+            }
         }
     }
 
@@ -117,10 +126,6 @@ private function getDefaultFromMethodReflection(\ReflectionMethod $reflection)
             return Generator::UNDEFINED;
         }
 
-        if (null === $param->getDefaultValue()) {
-            return Generator::UNDEFINED;
-        }
-
         return $param->getDefaultValue();
     }
 
@@ -136,10 +141,6 @@ public function getDefaultFromPropertyReflection(\ReflectionProperty $reflection
 
         $defaultValue = $reflection->getDeclaringClass()->getDefaultProperties()[$propertyName] ?? null;
 
-        if (null === $defaultValue) {
-            return Generator::UNDEFINED;
-        }
-
         return $defaultValue;
     }
 }