Set default value of toggle correctly

Use undefined instead of null so that the underlying reactstrap
component correctly sets the default value of toggle, and doesn't send
error messages to the console.

Author: tcbegley

src/components/popover/Popover.js

@@ -32,7 +32,7 @@ const Popover = props => {
       hideArrow={hide_arrow}
       // to ensure proper backwards compatibility, the toggle function is only
       // passed to the popover if `trigger` is not specified
-      toggle={trigger ? toggle : null}
+      toggle={trigger ? toggle : undefined}
       trigger={trigger}
       {...omit(['setProps'], otherProps)}
       data-dash-is-loading={