Plot.VerticalBar no longer forcibly includes zero in its x domain.

null is a falsy value :(

Need to add unit tests for this type of thing in general: Tracking
at #951.

Close #942.
Merge branch 'develop' into 942-verticalBar-zero
Apply fix to StackedBar and ClusteredBar.
Merge pull request #952 from palantir/942-verticalBar-zero

Plot.VerticalBar no longer forcibly includes zero in its x domain.