وَإِن مِّنْ أُمَّةٍ إِلَّا خَلَا فِيهَا نَذِيرٌ
The sentence is built on the pattern of negation + exception (in … illā), and in Arabic this structure conveys an all-embracing affirmation: to every community without exception.
I record this as an observation: within its own text the verse closes off the possibility of a community to which the knowledge never came. And this joins with the principle of the burden in the eighteenth verse of Fāṭir: responsibility is personal, and the warning is declared to have reached everywhere.
خَلَا — root خ-ل-و: to pass on, to become empty. The verb is in the perfect: "he came and went". That is, what is said is not that the warner is still present but that he came at some time.