`glucose_context` is nullable, but every surface that fanned out over the
four enum members treated the named list as exhaustive. A meter synced
through Apple Health writes no HealthKit meal-time metadata at all, so an
account can hold nothing but rows the list cannot match.
Add one shared notion of the untagged bucket — the name the Coach's glucose
block already used — with a resolver that files NULL, blank and unrecognised
values under it, and a grouper that walks the named contexts first and the
untagged bucket last. The untagged bucket borrows the RANDOM threshold: an
untagged reading is a spot reading, and a fifth threshold would give the user
a second editor row judging the same readings by a different number.
Labels in all seven locales.
Refs #943
Marc-André Bombeck
9a2b1f0955