PieChart follow-up bugfix: Handle 0 values properly
Bugfix in font handling of Pie, Bar and Line Charts and SuggestField

Bug was introduced in commit 665e98c90a292, when the test for `font == null`
was changed to `font === null`. However, font was never `null` but actually
`undefined`, which the changed comparison did not detect anymore.

Also fixed wrong constructor use in JS PieChart test.