Parcel hangs when PARCEL_WORKERS is set to zero

[RobertWHurst]

🐛 bug report

When the environment variable PARCEL_WORKERS is 0 parcel hangs on the first bundle.

🤔 Expected Behavior

Parcel should compile the bundle using the local worker in the worker farm.

😯 Current Behavior

Instead Parcel queues all work for remote workers even though none are spawned. This results in a hang.

💁 Possible Solution

Ensure the worker farm correctly uses the local worker instead of queuing for remote workers when no workers are spawned.

🔦 Context

Working on #1370, while debugging my changes, I ran into this issue.

🌍 Your Environment

Software	Version(s)
Parcel	Master
Node	v8.11.3
npm/Yarn	yarn v1.7.0
Operating System	macOS 10.13.5