!fixup https://github.com/go-gitea/gitea/pull/22177
The only place this function is used so far is in
findReadmeFileInEntries(), so the only visible effect of this oversight
was in an obscure README-related corner: if the README was in a
subfolder and was a symlink that pointed up, as in .github/README.md ->
../docs/old/setup.md, the README would fail to render when FollowLinks()
hit the nil ptree. This makes the ptree non-nil and thus repairs it.
This code was copy-pasted at some point. Revisit it to reunify it.
~~Doing that then encouraged simplifying the types of a couple of
related functions.~~
~~As a follow-up, move two helper functions, `isReadmeFile()` and
`isReadmeFileExtension()`, intimately tied to `findReadmeFile()`, in as
package-private.~~
Signed-off-by: Nick Guenther <nick.guenther@polymtl.ca>