Fix crash when a template type has zero typeattrs

[knoxsp]

Summary

• _bulk_add_resource_attrs stores tt.typeattrs into typeattr_lookup as-is. When a template type has zero typeattrs, that field serialises as None rather than [].
• The later typeattr_lookup.get(resource_type.id, []) fallback doesn't help, since the key exists (just mapped to None), so for ta in typeattrs raises TypeError: 'NoneType' object is not iterable.
• This hits any network containing a node/link whose type has no attributes at all (e.g. a plain junction).

Fix

Normalise to [] at the point of storage: typeattr_lookup[tt.id] = tt.typeattrs or [].

Test plan

• Reproduced against a local sqlite-backed hydra_base instance importing a network containing plain junction nodes; confirmed the crash without the fix and the successful import with it.