[BUG] env of "" should be treated as null/unset

[isaacs]

$ npm_config_only= npm config get only
npm WARN invalid config only="" set in environment
npm WARN invalid config Must be one of: null, dev, development, prod, production
null

Expect:

$ npm_config_only= node ../cli-v6 get only
null

[isaacs]

  loadEnv () {
    const prefix = 'npm_config_'
    const conf = Object.create(null)
    for (const [envKey, envVal] of Object.entries(this.env)) {
      if (!/^npm_config_/i.test(envKey) || envVal === '')  <-- edit this to add the check for ''
        continue
      const key = envKey.substr('npm_config_'.length)
        .replace(/(?!^)_/g, '-') // don't replace _ at the start of the key
        .toLowerCase()
      conf[key] = envVal
    }
    this[_loadObject](conf, 'env', 'environment')
  }