[BUG]: Mixed topology assembly (matrix) failing in parallel

### Summarize the issue

Running mixed topology mesh demo in parallel gives `NaN` in matrix, except on process 0.

### How to reproduce the bug

`demo_mixed-topology.py`

### Minimal Example (Python)

```Python

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### Output (Python)

```bash

```

### Version

main branch

### DOLFINx git commit

_No response_

### Installation

_No response_

### Additional information

_No response_