Don't create default folder when PARSE_SERVER_LOGS_FOLDER is null or "null"

[steven-supersolid]

While 'null' works for the logger adapter created in ParseServer.js, it does not work for the default logger that is created in logger.js.

E.g. currently if the following is set

export PARSE_SERVER_LOGS_FOLDER="null"

Then a folder with the name null is created.

[flovilmart]

Then a folder with the name null is created.

yeah the CLI is not nice, but if you put export PARSE_SERVER_LOGS_FOLDER= 

what does it do?

[steven-supersolid]

In that case the behaviour is the same with this code as master in that a folder named logs is created by default and the same folder is used by the non-default logger.

On Heroku I don't think it's possible to set an empty environment variable so the string null is useful.

Currently I'm using express mounted and passing a config option logsFolder: null but actually that doesn't avoid creating a folder either as defaults.js is still looking for the environment variable.