Fix Python error when the profile is outside of layer bounds

Currently if the selectd profile is completely out of bounds, the following error will pop up (because profile[nr]["z] is full of None values):

Traceback (most recent call last):
  File "/home/martin/.qgis2/python/plugins/profiletool/profileplugin.py", line 201, in doubleClicked
    self.doprofile.calculateProfil(self.pointstoDraw,self.mdl, self.plotlibrary)
  File "/home/martin/.qgis2/python/plugins/profiletool/tools/doprofile.py", line 80, in calculateProfil
    PlottingTool().reScalePlot(self.dockwidget.scaleSlider.value(), self.dockwidget, self.profiles, library)
  File "/home/martin/.qgis2/python/plugins/profiletool/tools/plottingtool.py", line 199, in reScalePlot
    if self.findMin(profiles, i,scale)   File "/home/martin/.qgis2/python/plugins/profiletool/tools/plottingtool.py", line 176, in findMin
    minVal = min( z for z in profiles[nr]["z"] if z is not None )
ValueError: min() arg is an empty sequence