concatenating with empty list changes dtype to float64

When concatenating anything with an empty list, the dtype of the result is augmented to `float64`.

MWE:
```python
import numpy

print(numpy.concatenate([[0]]).dtype)  # int64
print(numpy.concatenate([[0], [1]]).dtype)  # int64
print(numpy.concatenate([[0], []]).dtype)  # float64!
print(numpy.concatenate([[], [0]]).dtype)  # float64!
```