Fix Chunk::component_batch_raw not checking the bitmap first (#7286)

@abey79 discovered the issue when doing the Chunk Store UI.

Before:

![image](https://github.com/user-attachments/assets/3d117e0f-426a-4889-91da-81a8424aa4c7)

After:

![image](https://github.com/user-attachments/assets/aba46949-2fbc-442f-a5bd-0a7f3e9b11b4)

crates/store/re_chunk/src/helpers.rs

@@ -21,15 +21,17 @@ impl Chunk {
         component_name: &ComponentName,
         row_index: usize,
     ) -> Option<ChunkResult<Box<dyn ArrowArray>>> {
-        self.components.get(component_name).map(|list_array| {
+        self.components.get(component_name).and_then(|list_array| {
             if list_array.len() > row_index {
-                Ok(list_array.value(row_index))
+                list_array
+                    .is_valid(row_index)
+                    .then(|| Ok(list_array.value(row_index)))
             } else {
-                Err(crate::ChunkError::IndexOutOfBounds {
+                Some(Err(crate::ChunkError::IndexOutOfBounds {
                     kind: "row".to_owned(),
                     len: list_array.len(),
                     index: row_index,
-                })
+                }))
             }
         })
     }
@@ -258,7 +260,7 @@ impl UnitChunkShared {
         debug_assert!(self.num_rows() == 1);
         self.components
             .get(component_name)
-            .map(|list_array| list_array.value(0))
+            .and_then(|list_array| list_array.is_valid(0).then(|| list_array.value(0)))
     }
 
     /// Returns the deserialized data for the specified component.