Problems with poskin=False

Created by: lucarottoli

I have this issue if pos_use_kin is set to false

Traceback (most recent call last):
  File "/usr/local/bin/validphys", line 11, in <module>
    load_entry_point('validphys', 'console_scripts', 'validphys')()
  File "/Users/rottoli/nnpdf/nnpdfgit/nnpdf/validphys2/src/validphys/scripts/main.py", line 10, in main
    vp.main()
  File "/Users/rottoli/nnpdf/nnpdfgit/nnpdf/validphys2/src/validphys/app.py", line 131, in main
    a.main()
  File "/Users/rottoli/nnpdf/nnpdfgit/reportengine/src/reportengine/app.py", line 360, in main
    self.run()
  File "/Users/rottoli/nnpdf/nnpdfgit/nnpdf/validphys2/src/validphys/app.py", line 127, in run
    super().run()
  File "/Users/rottoli/nnpdf/nnpdfgit/reportengine/src/reportengine/app.py", line 346, in run
    rb.execute_sequential()
  File "/Users/rottoli/nnpdf/nnpdfgit/reportengine/src/reportengine/resourcebuilder.py", line 166, in execute_sequential
    *self.resolve_callargs(callspec))
  File "/Users/rottoli/nnpdf/nnpdfgit/reportengine/src/reportengine/resourcebuilder.py", line 173, in get_result
    fres =  function(**kwdict)
  File "/Users/rottoli/nnpdf/nnpdfgit/nnpdf/validphys2/src/validphys/plots.py", line 1126, in plot_positivity
    xvals - np.arange(ndata)
ValueError: operands could not be broadcast together with shapes (0,) (20,)