lint(track_config): fix error for missing track config

[ee7]

With the most recent configlet release (4.0.0-alpha.31), running
configlet lint using a directory that lacks a config.json file would
produce a segfault (!):

The lint command is under development.
Please re-run this command regularly to see if your track passes the latest linting rules.

Missing file:
/tmp/foo/config.json

SIGSEGV: Illegal storage access. (Attempt to read from nil?)

With this commit, we again get something like (for an empty directory):

The lint command is under development.
Please re-run this command regularly to see if your track passes the latest linting rules.

Missing file:
/tmp/foo/config.json

Missing directory:
/tmp/foo/exercises/shared/.docs

Missing directory:
/tmp/foo/docs

Configlet detected at least one problem.
For more information on resolving the problems, please see the documentation:
https://github.com/exercism/docs/blob/main/building/configlet/lint.md

The problem was introduced by commit e732894, and does not occur in
releases before 4.0.0-alpha.31.