From the literature on dying languages, the professionals say language die when they are not used in any domain, such as home, school, church, work, entertainment, etc.
The null domain is where only you are using the language and no one else is.
- Diary writing.
- Talking to the cat.
- Writing codes (e.g. your accounting books for income not reported to the tax authorities)
The null domain is for the language that isn’t ready for public use (like a new conlang), for when you’re studying a language outside of it’s normal range (Icelandic in Washington DC), or if you are in fact the last speaker of a rare language.
The null domain doesn’t really have a community, so in that respect, it isn’t fully a language. I would say it is a full language in the sense that it will evolve and be subject to the forces of erosion, sound shifts, etc.