Add check for default outdir to null

[maxulysse]

Description of feature

Example in the cutandrun: https://github.com/nf-core/cutandrun/blob/master/nextflow.config#L116

This params should really be set to null by default.

cf https://nfcore.slack.com/archives/C01FKFZ57SA/p1742382940814159

[FelixKrueger]

I created a PR to change the default name here: https://github.com/nf-core/cutandrun/pull/279

[edmundmiller]

hic still has it in master as well