val_is_null() bug

[NaN-git]

Tested with rapidyaml 0.7.2:

When parsing the yaml

{
  a: ,
  b:
}

then val_is_null() returns false for the node with key a, while it returns true for the node with key b. Both values are empty, i.e. they should be null.
Issue #259 can be parsed now, but it triggers the same behaviour.