[RFR] Button - don't display empty label and center icon if no label

[natrim]

If you provide no label to Button, then on mobile mode there is empty tooltip visible

and on desktop mode the icon is not center due to empty span with padding

this PR fixes that

btw. no idea if this should go to next or master, so made it for next first

[afilp]

I have noticed that too, thanks for the fix! I believe it could go to master also, but @fzaninotto has the final saying.

[fzaninotto]

Thanks!