[DowngradePhp72] Fix downgrade json null as true for DowngradeJsonDecodeNullAssociativeArgRector

[samsonasik]

@TomasVotruba this PR try to fix json_decode null arg, which it should be replaced to true instead of false as null and true means return associative array.

[samsonasik]

For possibly null, it seems needs to be handled with add if null check before use it:

+    if ($associative === null) {
+          $associative = true;        
+    }

    $value = json_decode($json, $associative);

[samsonasik]

@TomasVotruba it works 🎉

[TomasVotruba]

Awesome work, thank you 👍