Fix: specifying a skill lag of zero is now used.

[markscho]

Currently, when cast is performed, the skill/spell is queued with a lag value that is never zero, regardless of whether that lag is specified in the skill/spell or not. This is because zero is falsy, and I'm sure you were just trying to avoid pulling in the default lag value, which is null. The solution is to test whether the lag is currently null.

Additionally, there were no examples where lag was specified, which always resulted in using a default lag as found in Config. So I gave heal a default lag value so that developers are mindful of it.

[markscho]

I'm not sure where I thought spells/skills were being defaulted to null, but that was wrong. It should be undefined. This is defined in Skill.js.