Combobox popup height is non-zero when the model is empty

Steps:

• Open the example attached
• press "Ctrl + A" to output the popup height
• Toggle the popupType
• press "Ctrl + A" to output the popup height

Create a combobox in Universal style and set the model to 0. Open the menu. The height is 1px when setting popupType to PopupWindow, and 0 when set to PopupItem.

Failed test:

FAIL!  : tst_controls::Universal::ComboBox::test_emptyPopupAfterModelCleared() Compared values are not the same
   Actual   (): 1
   Expected (): 0
   Loc: [/home/edc/dev/qt/src/dev/qtdeclarative/tests/auto/quickcontrols/controls/data/tst_combobox.qml(1815)]