Trailing slash in Redis URL causes invalid db number

Created by: alexmic

Description

When running tests on Heroku CI, the in-dyno Redis URL environment variable is set as redis://h:xxx@localhost:6379/. The trailing slash causes this error: ReplyError: ERR invalid DB index, I presume because the url parser does not check if the / split actually yields a valid database number https://github.com/OptimalBits/bull/blob/1898b5a5b2a8e70d3bc5aaa85097cdf521b52bc9/lib/queue.js#L304

Bull version

3.14.0