Ignore optional undefined Json fields

[gedaiu]

No description provided.

[s-ludwig]

I was thinking about this and think that fixing this in readDictionary would solve a few additional cases that currently won't work as expected. So instead of handling undefined in tryReadNull, readDictionary would simply skip any undefined fields.

Similarly, readArray could also be patched to replace any undefined with null when setting m_current, to match the JavaScript JSON conversion semantics.

[s-ludwig]

Thanks!