Fix empty/null handling for Type.BaseType intrinsic

[sbomer]

@vitek-karas this should fix some of the issues you are seeing with dataflow in foreach loops involving Type.BaseType.

@vitek-karas shared the following testcase which was warning:

// Can only work with All annotation as NonPublicProperties doesn't propagate to base types
static void EnumeratePrivatePropertiesOnBaseTypesWithForeach ([DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.All)] Type type)
{
	const BindingFlags DeclaredOnlyLookup = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static | BindingFlags.DeclaredOnly;
	Type? t = type;
	while (t != null) {
		foreach (var p in t.GetProperties (DeclaredOnlyLookup)) {
			// Do nothing
		}
		t = t.BaseType;
	}
}

This was producing a warning because during the first pass of the analysis over t.BaseType, the analyzer hadn't yet figured out that t had annotation All. (The reason for this has to do with exception handling - the statement is reachable only through a finally block which we hadn't visited yet). BaseType gave back None, which produced a warning on the call to GetProperties.

What this indicates to me is that dotnet/runtime#106886 is important not just to be precise about the warning behavior when there are empty inputs, but also for the correctness of the solver.