OptionsDialog: do not NRE if CreatePanelWidget returns null
CombineMSBuildOptions: return null if MSBuild is required

If MSBuild is required for a project, previously the panel
widget would be an empty GTK Box. This caused two horizontal
lines in the UI to be displayed on top of each other that
would separate the panels, presumably because returning
null previously caused an NRE. The NRE is fixed, so return
null for a nicer UI.
Merge pull request #670 from mono/pr-abock-null-panel-widgets

Allow panel widgets to be null, fix ugly MSBuild build engine UI when MSBuild is required