API Restore nullable params

[emteknetnz]

Issue silverstripe/silverstripe-framework#10501

Few things to tidy up the recently merged config PR linked in the issue above:

• remove($class, $name = null) was existing functionality to remove an entire config, not just a single key/value for a class - default null paramter has been restroed
• Refactored string|null to the slightly shorter ?string syntax
• Converted a stray unserialize() method to __unserialize()
• Fixed a unit test changing merge() to set() because it was setting a non-array value