Clarify that DATABASE_URL can not be empty if allowlist is used

[abitrolly]

No description provided.

[boie0025]

This clarity would be helpful; I found myself in a situation with changed configuration, long story short, someone working on the project had previously set this particular application up used the default database.yml configuration with a change to use ENV.fetch(ENV[',...), which meant that DATABASE_URL wasn't set, but the configuration was setting the :url attribute from the fallback. The changes recently in configuration allow optionally pulling a .env.base in development/test from some common place, which set the DATABASE_URL to exactly the same thing as the fallback. And the configuration changes assumed pulling of the .env.base and set the whitelist in rails_helper.rb, leaving me astonished that I wasn't able to run specs because my DATABASE_URL wasn't whitelisted. Well of course, nil != /[regex]/, so I ultimately figured it out.

All of that said, it's also clear that a configuration using no DATABSE_URL environment variable wouldn't be protected by this, though I'm not sure how common that is in the wild anymore. (ref: #684)