Fix isPublic check for licenses array

[Hypfer]

As it turns out, not including bytecode is broken for modules that have a licenses array in their package.json.

One example of this is utils-merge which is a dependency of express.

Without this check, license will become undefined since there's no type property on that array.
This causes the module to be listed as non-public which means that it doesn't get included as CONTENT
That in turn triggers the exception when building without bytecode.

Anyways, this was not fun to debug, because there's no mention anywhere of this isPublic check.

[Hypfer]

Actually, this doesn't fix the issue #1139, because the module uid2, which is used in the example there, features no license at all and will therefore always be listed as private.

It doesn't matter for my main project and this still fixes a bug, however I suppose it would make sense to hide the isPublic check behind some kind of flag, so that it's just used by people that actually need it and not breaks foss projects for absolutely no reason

Edit:

As it turns out, there actually IS a flag that can disable that check. Unfortunately, there just doesn't seem to be any documentation about it anywhere.

--public-packages=packagea,packageb will disable the check for these two while --public-packages=* will disable the check altogether.

[robertsLando]

@Hypfer thanks for this fix, could you also mention this flag in docs please?

[Hypfer]

Done

[robertsLando]

LGTM