Do not include yarn.lock in upgrades

[t-sauer]

Right now yarn.lock is included in upgrades done via ember init. This is pretty unnecessary since the file is empty anyway and if you try to look at the diff it will take a couple of seconds.

@rwjblue suggested to check if a non empty yarn.lock file already exists, but in my opinion this could be annoying if you don't want to use yarn since the lockfile will appear again after every upgrade.

So for now I check if project.pkg.name is set which is the case for ember init in existing projects, but not for ember new or ember init in empty folders.

[rwjblue]

Thank you!