`make_seq2seq_fields` crashes with confusing error when empty prompt array is passed

## Description

### Bug Overview

The `make_seq2seq_fields` function in `gemma/gm/data/_functional.py` crashes with a confusing NumPy error when an empty prompt array is passed, instead of handling it gracefully or providing a clear error message.

### Bug Location

- **File**: `gemma/gm/data/_functional.py`
- **Line**: 142
- **Function**: `make_seq2seq_fields`

### Root Cause

The function attempts to create a target mask with this code:

```python
target_mask = np.concatenate([
np.zeros((len(prompt) - 1,), dtype=np.bool_),
np.ones((len(response),), dtype=np.bool_),
])
```

When `len(prompt) == 0`, the expression `len(prompt) - 1` evaluates to `-1`, causing `np.zeros((-1,), ...)` to raise:

```
ValueError: negative dimensions are not allowed
```

### Why This Is a Problem

1. **No input validation** - The function doesn't check for empty prompts before attempting array operations
2. **Confusing error message** - Users get a cryptic NumPy error instead of understanding what they did wrong
3. **Crashes entire pipeline** - Instead of handling the edge case gracefully, the entire operation fails
4. **Difficult to debug** - The error comes from deep within NumPy code, not from the user's code
5. **Poor user experience** - Users cannot easily identify that the issue is with an empty prompt

### How to Reproduce

**Scenario 1: Direct API usage**
```python
from gemma import gm

result = gm.data.make_seq2seq_fields(
prompt=[], # Empty prompt
response=[20, 21, 1]
)
# Raises: ValueError: negative dimensions are not allowed
```

**Scenario 2: Through AddSeq2SeqFields transform**
```python
from gemma import gm

transform = gm.data.AddSeq2SeqFields(
in_prompt="prompt",
in_response="response",
out_input="input",
out_target="target",
out_target_mask="target_mask",
)

element = {
"prompt": [], # Empty prompt tokens
"response": [20, 21, 1]
}

result = transform.map(element)
# Raises: ValueError: negative dimensions are not allowed
```

### Error Traceback

```
Traceback (most recent call last):
File ".../_functional.py", line 142, in make_seq2seq_fields
np.zeros((len(prompt) - 1,), dtype=np.bool_),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: negative dimensions are not allowed
```

### Expected Behavior

The function should either:
1. Raise a clear, descriptive error message indicating that empty prompts are not supported, OR
2. Handle empty prompts gracefully with a warning and continue execution

### Impact

- Affects users calling `make_seq2seq_fields` directly with empty prompt arrays
- Affects data pipelines using `AddSeq2SeqFields` transform with potentially empty prompt tokens
- Can cause entire batch processing pipelines to fail
- Difficult for users to identify and fix the root cause