MDDialog headline should be optional

[ddauer]

According to M3 design spec the headline is optional.

https://m3.material.io/components/dialogs/guidelines#0cff8b6b-d4f3-4442-8194-31212f5d1a12

However, when leaving the part out, the dialog retains its height (probably due to all height: self.minimum_height in dialog.kv, e.g.:

https://github.com/kivymd/KivyMD/blob/cfe5aac30f50708c6208c8b33b0ef1aa390721e8/kivymd/uix/dialog/dialog.kv#L34

Expected result: Only display provided content, e.g. the follow code should show no whitespaces but it does:

MDDialog(            
   MDDialogContentContainer(
      MDIcon(icon="weight")
   )
).open()

red areas indicate too much whitespace: