Better handling of undefined values

[pepone]

This PR fixes JavaScript OutputStream to better handling undefined values, in general, we treat undefined as not set optional value and null as an empty string or collection, this is not required in the methods that write nonoptional values, allowing these methods to accept undefined with the same semantic as null makes the code more flexible.

The attached file contains a prebuild of the ice npm package that includes this fix and also the previous fix for #1170

ice-3.7.5-pre.zip

[externl]

Looks good. I think we could also just use == instead of === as v == null will also work if v is undefined. However, your way is more explicit.

[pepone]

Looks good. I think we could also just use == instead of === as v == null will also work if v is undefined. However, your way is more explicit.

I prefer the explicit check, I think we have the linter setup to complain about == null, as we don't always want to treat undefined as null.

[bentoi]

Looks good!