Fix calculation of CVSSv3.1 environmental scores

[nscuro]

• Fixed environmental score calculation by using base score values if the environmental score values are null or NOT_DEFINED ("X") -> otherwise the environmental score is zero if not all fields are set
• Adds check for negative values in CvssV3#roundNearestTenth

Extracted from #17 by @michael-hinterdorfer.

Aligns CvssV3_1#calculateScore with the calculator from first.org (https://www.first.org/cvss/calculator/cvsscalc31.js).

Aside from fixing the score calculation, it also streamlines the calculation logic by removing unnecessary nesting.