If I have some field that I specify to go into the meta map in the samplesheet schema, but it is not required, does the meta map look like

[id: foo, field: null]

or

[id: foo]

if the field is not specified in the samplesheet?

[id: foo, field: null]

to maintain the structure of the channel. You can change the default value with the

default

keyword (https://nextflow-io.github.io/nf-schema/latest/nextflow_schema/nextflow_schema_specification/#default)

Phew, that's what I was hoping for!