How do I use a contextual filter to find all nodes with field_abc that has a null value?
1) Views PHP Filter is good to go when you have 'not that many' query results in total. The thing is that it loads node's object and it's not the best choice if you opt for better performance, but still it's okay for quick views filters creation.
2) Create new Computed field and store any flag (1/0 is quite enough) as its value. The flag itself will indicate if the field X has any content and then it can be used by Views for filtering.