fix: return empty list instead of null when user explicitly sets []

[BohdanTkachenko]

Previously, when a user set an empty list like hostdevs = [], the provider returned null instead of [] after apply, causing "Provider produced inconsistent result after apply" errors.

The fix adds a check: when the plan has an explicitly set (non-null) list but the XML has no elements, return an empty list to preserve user intent. This applies to both nested object lists and simple value lists.

• Import/datasource (no plan): still returns null for absent data
• User set field = []: now correctly returns []
• User set field = null or omitted: returns null

Fixes #1247