Fix "empty buffer" warnings and potential empty chunks

doc/source/changelog.md

@@ -17,6 +17,7 @@ Primarily developped with Godot 4.3.
     - `VoxelTerrain`: edits and copies across fixed bounds no longer behave as if terrain generates beyond (was causing "walls" to appear).
     - `VoxelGeneratorGraph`: fix wrong values when using `OutputWeight` with optimized execution map enabled, when weights are determined to be locally constant
     - `VoxelMesherTransvoxel`: revert texturing logic that attempted to prevent air voxels from contributing, but was lowering quality. It is now optional as an experimental property.
+    - `VoxelStreamSQLite`: Fixed "empty size" errors when loading areas with edited `VoxelInstancer` data
 
 - Breaking changes
     - `VoxelInstanceLibrary`: Items should no longer be accessed using generated properties (`item1`, `item2` etc). Use `get_item` instead.

streams/voxel_stream_cache.cpp

@@ -14,13 +14,17 @@ bool VoxelStreamCache::load_voxel_block(Vector3i position, uint8_t lod_index, Vo
 		return false;
 
 	} else {
+		const Block &block = it->second;
+		if (!block.has_voxels) {
+			// Has a block in cache but there is no voxel data
+			return false;
+		}
 		// In cache, serve it
 
-		const VoxelBuffer &vb = it->second.voxels;
 
 		// Copying is required since the cache has ownership on its data,
 		// and the requests wants us to populate the buffer it provides
-		vb.copy_to(out_voxels, true);
+		block.voxels.copy_to(out_voxels, true);
 
 		return true;
 	}
@@ -31,6 +35,10 @@ void VoxelStreamCache::save_voxel_block(Vector3i position, uint8_t lod_index, Vo
 	RWLockWrite wlock(lod.rw_lock);
 	auto it = lod.blocks.find(position);
 
+	ZN_ASSERT_RETURN_MSG(
+			!Vector3iUtil::is_empty_size(voxels.get_size()), "Saving voxel buffer with empty size is not expected. Bug?"
+	);
+
 	if (it == lod.blocks.end()) {
 		// Not cached yet, create an entry
 		Block b;