ValueBox without subtitle or footer

[muschellij2]

My use case was to have a ValueBox without a subtitle or footer.

The issue is that even when you specify subtitle = NULL, you still will get <p class="small-box-subtitle"></p> as shiny::tags$p(class = "small-box-subtitle", subtitle) will still result in a tag, which is executed at:
https://github.com/RinteRface/bs4Dash/blob/master/R/cards.R#L778

Also, with the default footer = NULL, there is space that is added to the valuebox that is not desirable:
https://github.com/RinteRface/bs4Dash/blob/master/R/cards.R#L805

I have added a PR at #289 to fix.