[RN] Metro config is no longer optional

[janicduplessis]

RN CI currently fails with

(node:3341) UnhandledPromiseRejectionWarning: Error: Could not find configuration for metro, did you create a 'metro.config.js'?
    at /private/tmp/react-native-8DEOpVT4/EndToEndTest/node_modules/metro-config/src/loadConfig.js:139:13
    at Generator.next (<anonymous>)
    at step (/private/tmp/react-native-8DEOpVT4/EndToEndTest/node_modules/metro-config/src/loadConfig.js:253:447)
    at /private/tmp/react-native-8DEOpVT4/EndToEndTest/node_modules/metro-config/src/loadConfig.js:253:607
    at <anonymous>

Not sure if this is intentional or not but I think it should still work without a config file.

cc @CompuIves @rafeca

[kelset]

Possibly related? react/react-native@140b230

[rafeca]

@CompuIves has a fix for this which is currently landing ✌️