Fixes three issues in bundle.go / bundle_test.go:
1. Missing sqlmock import: TestBundleImport_ValidJSON and
TestBundleExport_NotFound use sqlmock.Sqlmock from setupTestDB()
and call sqlmock.NewResult() but did not import go-sqlmock,
causing a build failure.
2. Empty/null bundle guard: null JSON (ShouldBindJSON → zero-value Bundle{})
or empty {} payload would bind without error and reach bundle.Import(),
INSERTing a row with name="" and tier=0 into workspaces before
failing. Add b.Schema != "" guard before calling bundle.Import().
3. Outdated test expectations: TestBundleImport_ValidJSON expected
INSERT INTO workspace_schedules and workspace_secrets which the current
importer does not issue. Remove those expectations so the test
reflects actual importer behaviour (INSERT + UPDATE runtime only).
Closes#850
