NULL input for rhandsontable

[nick-youngblut]

At least for handsontable 0.3.8, providing NULL as input for rhandsontable() produces the error:

Error in get_col_types: Unsupported object type: NULL Can't extract column types.

Why not just add a simple check at the start of the function of if(is.null(df)) return(NULL)? This would make the UI more aligned with DT::renderDataTable() and the UI of various shiny functions.