Non pareil fails if data is insufficient for calculation due to a missing png

[jfy133]

Description of the bug

The tool itself odesn't fail (exit code 0) but no png is made and module says it's not optional.

We should update the non-pareil curve module to make the png optional

Thanks to @Joon-Klaps for reporting here: nf-core/detaxizer#39 (comment)

Command used and terminal output

No response

Relevant files

No response

System information

No response

[jfy133]

Reopening until in the pipeline