browserlist.coverage fail with custom coverage file

[danylkaaa]

Hello! I tried to use your snippet to get coverage when querying my custom stats.

// process.env.BROWSERSLIST_STATS === path.join(__dirname, './browserslist-stats.json');
const browsers = browserslist();
browserslist.coverage(browsers, 'my stats')

But I get this error

TypeError: Cannot read property 'op_mini' of undefined
    at normalizeStats (<...>/node_modules/browserslist/node.js:127:11)
    at Object.getStat (<...>/node_modules/browserslist/node.js:209:12)
    at Function.browserslist.coverage (<....>/node_modules/browserslist/index.js:580:27)

I found that it is caused by this 3f54ebd/index.js#L580 :

// node_modules/browserslist/index.js:580
var customStats = env.getStat(opts)

Which calls getStat() from browserslist/node.js

3f54ebd/node.js#L209

  getStat: function getStat (opts, data) {
    // some ops, but "data" argument is not overrided
    return normalizeStats(data, stats) // <- data is undefined
  }

And then browserlist tries to access the data members w/o checking that the data exists.

3f54ebd/node.js#L127

if (
      versions.length === 1 &&
      data[i] && // <- raises error
      data[i].versions.length === 1
    )

So, I considered to add extra check

function normalizeStats (data, stats) {
  if (!data) {
    data = {};
  }
  //...
}

[danylkaaa]

Also I created PR #608 with my proposed changes