gen_stub: simplify ArgInfo::getDefaultValueAsMethodSynopsisString()

There is no need to add special handling for the default value of `null`, since
it is not loosely-equals to any of the strings 'UNKNOWN', 'false', 'true', or
'null' it will just be returned directly anyway.

Author: DanielEScherzer

build/gen_stub.php

@@ -848,10 +848,6 @@ public function getDefaultValueAsArginfoString(): string {
     }
 
     public function getDefaultValueAsMethodSynopsisString(): ?string {
-        if ($this->defaultValue === null) {
-            return null;
-        }
-
         switch ($this->defaultValue) {
             case 'UNKNOWN':
                 return null;