The test runner walks link-field dependencies recursively to pre-generate
test records via `get_missing_records_doctypes`. If any DocType in the
transitive link graph belonged to an app not installed on the test site,
the walk crashed with `DoesNotExistError`, aborting the entire suite
before a single test ran.
Treat such link targets as dead-end leaves instead:
- `get_modules` now returns `(None, None)` when the DocType row does not
exist, instead of falling through into `load_doctype_module` which
raises.
- `get_missing_records_doctypes` checks for `module is None`, logs a
warning naming the parent DocType that linked to it, and returns
without descending further.
This restores the ability to run downstream test suites that link
(directly or transitively) to optional/uninstalled apps without forcing
every CI environment to know the full transitive link graph.
Fixes#38747
38e140df22