ELN file minor issues #1593

Just some minor tweaks required to our implementation of the ELN format, raised in TheELNConsortium/TheELNFileFormat#136:

• null values for ro-crate fields (e.g., dateCreated) should be omitted rather than set to null
• missing license for root dataset
• missing version for ELN file context
• missing entries for File types
• missing entries for Person types for authorship
• missing Software entry with version for datalab
• missing purl for datalab entry