clean_figure() error for plot() line outside the box

```python
	import matplotlib.pyplot as plt
	import tikzplotlib
	plt.xlim(0, 1)
	plt.plot([5, 6], [2, 3])
	tikzplotlib.clean_figure()
```

Error: 

```
IndexError: index 0 is out of bounds for axis 0 with size 0
```

At least for `scatter` instead of `plot` it doesn't give the error.