[Fix] order: Recognize pathGroup config for first group

[forivall]

With the config

  'import/order': [
    'warn',
    {
      groups: ['builtin', 'external', 'internal', 'parent', 'sibling', 'index'],
      alphabetize: {
        order: 'asc',
      },
      pathGroups: [{ pattern: 'react', group: 'builtin' }],
      pathGroupsExcludedImportTypes: ['builtin'],
    },
  ],

, react wasn't being recognized as a part of the "builtin" group, since the rank would end up being 0, which is a falsy value. The fix here, is to check for undefined instead.

My current workaround is to define

      pathGroups: [{ pattern: 'react', group: 'builtin', position: 'before' }],

instead, (which results in a rank -0.1, so it's not falsy).

[coveralls]

Coverage remained the same at 97.797% when pulling 2171443 on forivall:fix/first-path-group-setting-ignored into 71ca88f on benmosher:master.

[coveralls]

Coverage remained the same at 97.797% when pulling 2171443 on forivall:fix/first-path-group-setting-ignored into 71ca88f on benmosher:master.

[coveralls]

Coverage remained the same at 97.797% when pulling 2171443 on forivall:fix/first-path-group-setting-ignored into 71ca88f on benmosher:master.

[coveralls]

Coverage remained the same at 97.797% when pulling 2171443 on forivall:fix/first-path-group-setting-ignored into 71ca88f on benmosher:master.

[coveralls]

Coverage remained the same at 97.797% when pulling 2171443 on forivall:fix/first-path-group-setting-ignored into 71ca88f on benmosher:master.

[ljharb]

Combined this and the test from #1724. Thanks!

[ljharb]

Because I pushed this to two PRs, two appveyor builds ran. The second failed (https://ci.appveyor.com/project/benmosher/eslint-plugin-import/builds/32116513), but the first succeeded (https://ci.appveyor.com/project/benmosher/eslint-plugin-import/builds/32116512), so I'm going to merge through the failure.