Core: Fix config.js-based configuration

[shilman]

Issue: #14458

What I did

Don't try to require main.js when the file doesn't exist

self-merging @ndelangen

How to test

cd examples/react-ts
# replace main.js with config.js
yarn storybook 

// config.js
import { configure } from '@storybook/react';
configure(require.context('./src', true, /\.stories\..*/), module);

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