Regression: Converter on nilable field in JSON::Serializable fails when reading null value

[straight-shoota]

Some of the refactoring in JSON::Serializable (probably #13433) breaks reading null values on a nilable field when using a converter.

The following code works fine in 1.8.2 but raises in 1.9.0:

require "json"

class Foo
  include JSON::Serializable
  @[JSON::Field(converter: Time::EpochConverter)]
  property bar : Time?
end

Foo.from_json((%({"bar": null})) # JSON::SerializableError: Expected Int but was Null at line 1, column 13 parsing Foo#bar at line 1, column 2

Fields without a converter are unaffected because the deserializer tries to deserialze each of the union types. That includes Nil and it succeeds returning nil.

This was discovered in https://forum.crystal-lang.org/t/json-serializable-converters-and-nilable-values/5828

A potential workaround is to wrap the converter with explicit null handling:

require "json"

module NilableConverter(T)
  def self.from_json(parser : JSON::PullParser)
    parser.read_null_or do
      T.from_json(parser)
    end
  end
end

class Foo
  include JSON::Serializable
  @[JSON::Field(converter: NilableConverter(Time::EpochConverter))]
  property bar : Time?
end

Foo.from_json((%({"bar": null}))

Perhaps this would even be a better way to implement this, being more explicit and giving full control of null handling to the converter.
But it used to work that null are implicitly read and turned in nil for nilable fields, so this should be restored.