fix: avoid crashes on none values

[1337MARCEL]

Downloading https://wiki.leagueoflegends.com/en-us/Template:Data_Heimerdinger/Hextech_Rocket_Swarm
ERROR: FAILURE TO PARSE MODIFIER:  Initial Rocket Damage:
ERROR: Could not parse a simple flat value: Initial Rocket Damage:
Traceback (most recent call last):
  File "C:\Coding\temp\lolstaticdata\lolstaticdata\champions\pull_champions_wiki.py", line 724, in _render_modifiers
    modifier = self._render_modifier(lvling, nvalues)