DSL2: Print Nuclear Contamination JSON N/A value

[TCLamnidis]

The print nuclear contaminatino script produces a JSON with the results, in which the na value is set to "N/A". This should be replaced with a valid JSON missing value (null?) so that loading of the JSON for downstream processing does not raise errors (strings vs floats).

[TCLamnidis]

See multimeric/TidyMultiqc#9 for a similar issue with NaNs

[scarlhoff]

As of #1004, we now print "none" in the nuclear_contamination.txt and "null" in the nuclear_contamination.json. Will need to observe how MultiQC behaves