`srfi/64` fails on an empty test suite

[rvs314]

The following program fails:

#lang racket

(require srfi/64)

(test-begin "foo")
(test-end)

with the following error:

assq: not a proper list: 0
Context (plain):
 PATH_TO/racket/pkgs/srfi-lib/srfi/64/testing.rkt:309:2 %test-format-line
 PATH_TO/racket/pkgs/srfi-lib/srfi/64/testing.rkt:319:2 %test-end
 PATH_TO/elpa/racket-mode-20250828.1507/racket/repl.rkt:323:0 configure/require/enter

It seems like the error occurs here: https://github.com/racket/srfi/blob/12213e013937b0c1532de2238cf04906f589b302/srfi-lib/srfi/64/testing.rkt#L322C19-L322C36
A simple fix would be to just add a case to test for when line-info is 0, then return an empty string in such a case. It might also be possible to fill the slot in earlier in %test-end with potentially meaningful data.