fix: forge skills

[luanluciano93]

• fix error:

ERROR: Lua exception: /game_skills/skills.lua:104: attempt to index local 'skill' (a nil value)
stack traceback:
    [C]: in function '__index'
    /game_skills/skills.lua:104: in function 'setSkillBase'
    /game_skills/skills.lua:512: in function </game_skills/skills.lua:511>
ERROR: protected lua call failed: LUA ERROR:
/game_skills/skills.lua:104: attempt to index local 'skill' (a nil value)
stack traceback:
    [C]: in function '__index'
    /game_skills/skills.lua:104: in function 'setSkillBase'
    /game_skills/skills.lua:512: in function </game_skills/skills.lua:511>

• removed redundancy in onBaseSkillChange's refresh() function
  
  

• readjustment for forge skills only appear when they are greater than 0

transcendencia.mp4

[kojimhojo]

this is displayed on older servers (8.6) , there should be an if of x version @luanluciano93

[luanluciano93]

this is displayed on older servers (8.6) , there should be an if of x version @luanluciano93

test plx https://github.com/mehah/otclient/pull/784/files