Call default value method only if null value was parsed for the field

[plokhotnyuk]

Currently it is called for all fields in the start of decoding method, for example:

private def d0(in: JsonReader, default: JsonCodecMakerSpec.this.Defaults): JsonCodecMakerSpec.this.Defaults = if (in.isNextToken('{')) {
  var _s: String = Defaults.`<init>$default$1`;
...
  _s = in.readString(_s)

Should be:

private def d0(in: JsonReader, default: JsonCodecMakerSpec.this.Defaults): JsonCodecMakerSpec.this.Defaults = if (in.isNextToken('{')) {
  var _s: String = null;
...
    _s = in.readString(_s)
    if (_s == null) _s = Defaults.`<init>$default$1`;

Or use presence bits in p0, p1, ... , pN variables instead checking for null.

[plokhotnyuk]

Related to refactoring required for adding support of default values for non-first list of arguments of primary constructors: https://github.com/plokhotnyuk/jsoniter-scala/issues/904