Change from null checks to coercive truthiness checks on non-primitive p...

...arts of the public API.

Rationale: This will be a little more idiomatic for javascript consumers without changing semantics, since
an object (ie. any non primitive) type will always return true if it is defined.

This would enable usage like:
`new Plottable.Component.Gridlines(xGridlinesEnabled && xScale, yGridlinesEnabled && yScale)`