NullRef in ValidateProjectHasTarget

[KirillOsenkov]

When typing this document:

<Project>
 <
</Project>

seeing this exception:

	MonoDevelop.MSBuild	MSBuildDocumentValidator.ValidateProjectHasTarget Line 172
	MonoDevelop.MSBuild	MSBuildDocumentValidator.ValidateResolvedElement Line 73
	MonoDevelop.MSBuild	MSBuildDocumentValidator.VisitResolvedElement Line 44
	MonoDevelop.MSBuild	MSBuildVisitor.ResolveAndVisit Line 74
	MonoDevelop.MSBuild	MSBuildVisitor.Run Line 55
	MonoDevelop.MSBuild	MSBuildVisitor.Run Line 42
	MonoDevelop.MSBuild	MSBuildRootDocument.Parse Line 198