text: Title: clean_figure() error for plot() line outside the box
  Body: ```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.