Return meaningful error when no devices available
Return meaningful error when no devices available

Summary:
`react-native run-ios --device` should report 'No iOS devices connected.' in case when no devices attached. However due to logic bug when empty array is being treatened as a boolean value, cli reports odd message, saying `Could not find device with the name: "true".`

Disconnect all iOS devices from developer machine and run `react-native run-ios --device` in any project - console message should state that there are no devices available.
Closes https://github.com/facebook/react-native/pull/15603

Differential Revision: D5686891

Pulled By: shergin

fbshipit-source-id: 289461b6f920691ad39e940ecca6d85cf2a3573b

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