[partition] Centralize setting-of-mountpoint

- map empty to the 0'th index
 - add new entries as needed

This avoids having selected index 0, but a different text.

src/modules/partition/gui/CreatePartitionDialog.cpp

@@ -328,7 +328,7 @@ CreatePartitionDialog::initFromPartitionToCreate( Partition* partition )
     m_ui->fsComboBox->setCurrentText( FileSystem::nameForType( fsType ) );
 
     // Mount point
-    m_ui->mountPointComboBox->setCurrentText( PartitionInfo::mountPoint( partition ) );
+    setSelectedMountPoint( m_ui->mountPointComboBox, PartitionInfo::mountPoint( partition ) );
 
     updateMountPointUi();
 }