Avoid stacking when stack options are not defined

When stacking plugin is included in the page but `stack: ...` options in `series` aren't defined, Flot attempts to stack the charts.

Following fix correctly avoids stacking for values `undefined` along with existing `null` and `false`. It however does not reject `0` (as expected).

__Note__: The seemingly hacky syntax (`((s.stack+"") !== "0")`) is actually to prevent jshint errors caused on using double equal to `==`