Tryptophan Operon Working Mechanism
You're right that if there was really a complete lack of Trp, then the cell wouldn't be able to make the enzymes. But the cell doesn't have to wait until [Trp] drops all the way to zero -- it can induce expression of the operon when the concentration gets sufficiently low (in practice there's some continous relationship between [Trp] and expression). Note that there's always going to be a little bit of Trp around as there are constantly proteins being degraded (and resynthesized).

But you're on to something -- in fact there is some bias towards lower usage of a particular amino acid in enzymes that are responsible for synthesizing that amino acid. Here's the paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1839009/ although the effect isn't that high for Tryptophan.