fix: ensure consistent empty repository topics field (#7920)

Resolves #7878

An empty repository topic was not stored consistently across databases, this caused the `ONLY_SHOW_RELEVANT_REPOS` feature to not work correctly. Always store empty repository topics as an empty array to fix this.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7920
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Maks1mS <maks1ms@noreply.codeberg.org>
Co-committed-by: Maks1mS <maks1ms@noreply.codeberg.org>

models/forgejo_migrations/v31_test.go

@@ -0,0 +1,38 @@
+// Copyright 2025 The Forgejo Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package forgejo_migrations //nolint:revive
+
+import (
+	"testing"
+
+	migration_tests "forgejo.org/models/migrations/test"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_SetTopicsAsEmptySlice(t *testing.T) {
+	type Repository struct {
+		ID     int64    `xorm:"pk autoincr"`
+		Topics []string `xorm:"TEXT JSON"`
+	}
+
+	x, deferable := migration_tests.PrepareTestEnv(t, 0, new(Repository))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	require.NoError(t, SetTopicsAsEmptySlice(x))
+
+	var repos []Repository
+	require.NoError(t, x.Find(&repos))
+
+	for _, repo := range repos {
+		if repo.ID == 2 {
+			require.Equal(t, []string{"go", "dev"}, repo.Topics, "Valid topics should remain unchanged")
+		} else {
+			require.Equal(t, []string{}, repo.Topics, "NULL topics should be set to empty array")
+		}
+	}
+}