fix(revit): guard against null Zone in space send and receive

[MayconAlvesss]

What does this PR do?

Resolves a two-part bug that caused the Revit connector to fail when receiving spaces that have no zone assigned.

Closes #3524

Root cause

Space.Zone returns null from the Revit API when a space is not assigned to any zone. The converter had two unguarded references to this property:

1. On send (SpaceToSpeckle)

// Before: NullReferenceException when Space.Zone is null
speckleSpace.zone = cache.GetOrAdd(revitSpace.Zone.Name, ...);

If revitSpace.Zone is null, accessing .Name throws immediately and the whole send operation fails.

2. On receive (SpaceToNative)
The zone association block ran unconditionally. When the incoming speckleSpace.zone was null (correctly serialized after this fix, or received from another host app without zone data), CreateRevitZoneIfNeeded returned the Revit document's internal default zone object. The connector then attempted AddSpaces on it, which triggered an API error — and in the error recovery path, the existing spaces were deleted instead of updated.

Changes

• ConvertSpace.cs — SpaceToSpeckle: wrapped the cache/serialize call in a null guard so spaces without a zone serialize with zone = null rather than throwing.
• ConvertSpace.cs — SpaceToNative: moved the entire zone-association block inside a speckleSpace.zone != null check. When no zone was intended, the connector leaves the space's zone state untouched.

Testing

Manually traced through the execution path with a Space mock where Zone is null. The updated logic skips the zone block cleanly in both directions. No changes to the happy path (spaces with zones) — the logic is structurally identical to before for those cases.