render plain text file if the LFS object doesn't exist (#31812)

We had an issue where a repo was using LFS to store a file, but the user
did not push the file. When trying to view the file, Gitea returned a
500 HTTP status code referencing `ErrLFSObjectNotExist`. It appears the
intent was the render this file as plain text, but the conditional was
flipped. I've also added a test to verify that the file is rendered as
plain text.

routers/web/repo/view.go

@@ -234,14 +234,12 @@ func getFileReader(ctx gocontext.Context, repoID int64, blob *git.Blob) ([]byte,
 	}
 
 	meta, err := git_model.GetLFSMetaObjectByOid(ctx, repoID, pointer.Oid)
-	if err != nil && err != git_model.ErrLFSObjectNotExist { // fallback to plain file
+	if err != nil { // fallback to plain file
+		log.Warn("Unable to access LFS pointer %s in repo %d: %v", pointer.Oid, repoID, err)
 		return buf, dataRc, &fileInfo{isTextFile, false, blob.Size(), nil, st}, nil
 	}
 
 	dataRc.Close()
-	if err != nil {
-		return nil, nil, nil, err
-	}
 
 	dataRc, err = lfs.ReadMetaObject(pointer)
 	if err != nil {

tests/gitea-repositories-meta/user2/lfs.git/objects/30/77e1c4c8964613df72c37d14275c1eda5228a9

@@ -0,0 +1,2 @@
+x
KÊÉOR0´0`pö÷	òt
+
ñôs×ËMQHËÌ)I-²ÍI+VHÉLK3rS‹ÒSÁ,Ý’ÔŠ.-½¬‚t"U&eæ¥23¯,1'“8û
ØæAÅ

tests/gitea-repositories-meta/user2/lfs.git/refs/heads/master

@@ -1 +1 @@
-73cf03db6ece34e12bf91e8853dc58f678f2f82d
+e9c32647bab825977942598c0efa415de300304b

tests/integration/lfs_view_test.go

@@ -89,6 +89,19 @@ func TestLFSRender(t *testing.T) {
 		content := doc.Find("div.file-view").Text()
 		assert.Contains(t, content, "Testing READMEs in LFS")
 	})
+
+	// check that an invalid lfs entry defaults to plaintext
+	t.Run("Invalid", func(t *testing.T) {
+		defer tests.PrintCurrentTest(t)()
+
+		req := NewRequest(t, "GET", "/user2/lfs/src/branch/master/invalid")
+		resp := session.MakeRequest(t, req, http.StatusOK)
+
+		doc := NewHTMLParser(t, resp.Body).doc
+
+		content := doc.Find("div.file-view").Text()
+		assert.Contains(t, content, "oid sha256:9d178b5f15046343fd32f451df93acc2bdd9e6373be478b968e4cad6b6647351")
+	})
 }
 
 // TestLFSLockView tests the LFS lock view on settings page of repositories