Empty describe block throws error

🐛 Bug Report

If a describe block is left empty, with no tests passed to it, an error is thrown because the list of tests is not defined. Whether this is valid or not, it should probably return a specific message instead of a cryptic runtime error.

If I have an empty describe like:

describe('Scenario needing some tests')

It will fail with Cannot read property 'length' of undefined:

  ● Test suite failed to run

    TypeError: Cannot read property 'length' of undefined

      at Env.describe (node_modules/jest-jasmine2/build/jasmine/Env.js:297:27)
      at Object.<anonymous> (client/test/components/AccountChooser/AccountChooser-test.js:10:1)
          at Generator.next (<anonymous>)
          at new Promise (<anonymous>)
          at Generator.next (<anonymous>)
          at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:188:7)

Personally I find the workflow of creating a describe block for every set of tests I know I will need up front, and then coming back and filling them in as I get time, to be really useful. Some may argue that it should be considered invalid, but in that case it should throw an error explaining as much.

To Reproduce

Just create a describe instance with only a name param supplied

Expected behavior

Either no error at all, or a friendly error message like ‘Every describe block requires some tests’

Link to repl or repo (highly encouraged)

https://repl.it/repls/UnselfishAmusedNanocad

Run npx envinfo --preset jest

Paste the results here:

[SimenB]

This is basically #5558, but for describe instead of it/test. Wanna send a PR for it?

[github-actions[bot]]

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