Sometimes information is missing. Maybe it was never collected, maybe it was wrong. Whatever the case, let blanks be blank.
Don’t use placeholders when you mean “I don’t know”. Using abbreviations like “N/K” or even words like “Unknown” can seem helpful but are only really useful for humans who speak your language (and understand what the abbreviation really means!)
Placeholders make it harder to work out that there is actually something missing. Think about all the ways you can write “Not Known”! It’s far better to leave a blank. Blanks are familiar and can be picked up by lots of tools.
Tip: Blanks are fine for numbers too.
See all the tips in one place: Good Quality Open Data