rnafusion fails without displaying the right error when outdir is not provided

[nschcolnicov]

Running this command that doesn't specify the --outdir parameter:
nextflow run nf-core/rnafusion -r dev -profile docker,test
Raises this error:

ERROR ~ Unknown method invocation `getFileSystem` on LinkedList type

When instead, it should raise this other error:

ERROR ~ Validation of pipeline parameters failed!

 -- Check '.nextflow.log' file for details
The following invalid input values have been detected:

* Missing required parameter(s): outdir