Fix TypeError in case no files were found

[skovhus]

In case no files are found then jscodeshift crashes with a TypeError as pendingWorkers are undefined, so Promise.all(pendingWorkers) will fail.

Output before this PR:

No files selected, nothing to do. 
(node:56389) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
    at Function.all (<anonymous>)
    at getAllFiles.then.then.pendingWorkers (/node_modules/jscodeshift/src/Runner.js:292:17)
(node:56389) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
(node:56389) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

After this PR the output looks as expected:

No files selected, nothing to do. 
All done. 
Results: 
0 errors
0 unmodified
0 skipped
0 ok

Alternatives

I was considering if we should simply do a process.exit, but that wouldn't work if people use jscodeshift directly as a node dependency. I think the proposed solution here is the simplest for now.

[skovhus]

@fkling anything I can do to land this?

[fkling]

Sorry for the wait!