[Accessibility] Provide default alt value for `plotTag()`

[jooyoungseo]

plotTag() has alt param for accessibility, which is great; however, it does not provide default value.

Please provide default value as "Plot object" so that any NULL-alt plots can be also recognized by screen readers.

If possible, please add a condition to check whether current graph is ggplot2 object having alt attribute so that g$labels$alt can be passed to alt param when available.

[jooyoungseo]

Oops, it should be reported in htmltools.