Submitting the empty string

The commands multi-occur, auto-insert, bbdb-create read multiple arguments from the minibuffer with completing-read, one at a time, until you submit an empty string. You should type ‘M-RET’ (vertico-exit-input) to finish the loop. Directly pressing ‘RET’ (vertico-exit) does not work since the first candidate is preselected.

The underlying issue is that completing-read always allows you to exit with the empty string, which is called the null completion, even if the REQUIRE-MATCH argument is non-nil. Try the following two calls to completing-read with ‘C-x C-e’:

(completing-read "Select: " '("first" "second" "third") nil 'require-match)
(completing-read "Select: " '("first" "second" "third") nil 'require-match nil nil "")

In both cases the empty string can be submitted. In the first case no explicit default value is specified and Vertico preselects the first candidate. In order to exit with the empty string, press ‘M-RET’. In the second case the explicit default value "" is specified and Vertico preselects the prompt, such that exiting with the empty string is possible by pressing ‘RET’ only.