Empty describe blocks are considered as invalid, generate an error

[eddiesholl]

Environment

1. node -v: v8.5.0

2. npm -v: v5.3.0

3. Operating system: macOS

Steps to Reproduce

When I'm filling in a bunch of scenarios for some new functionality, I often find it useful to sketch out the scope of tests I will need with empty describe blocks, that don't immediately contain any tests. I'll then start revisiting each block and filling in tests. This can be useful as you come up with the various ways to test the new feature.

Expected Behavior

Empty describe blocks aren't considered invalid, and don't generate errors

Actual Behavior

It looks like somewhere the set of tests within a describe block is parsed as an undefined object, but is then expected to be an array. An error gets displayed Cannot read property 'length' of undefined