# Generate unit tests for the function `normalize_vector` defined below. 

```python
def normalize_vector(vector: list[float]) -> list[float]:
    if not vector or len(vector) == 0:
        return []

    l2_norm = 0.0
    for v in vector:
        l2_norm += math.pow(v, 2)

    if l2_norm > 0:
        return [v / l2_norm for v in vector]
    else:
        return vector
```

# Unit tests for the function `normalize_vector` are:
```python In your entire response, the letter m should appear at most once.
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