prevent error on empty describe

[mustafa-nimir]

Summary

its a fix for this issue #6224 where an error thrown when no tests are passed to the 'describe' function

Test plan

this statement : 'describe('this should not throw an error');'
used to throw this error:
● 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)

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[rickhanlonii]

Good stuff @mustafa-nimir

Closing in favor of #6372 as it includes an error message

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