config/hcl2shim: don't put nils in ConfigValue maps

Absent values are omitted by the old code we are emulating in HCL, so we
must do the same here in order to avoid breaking assumptions in the
helper/schema layer.

config/hcl2shim/values.go

@@ -73,7 +73,10 @@ func ConfigValueFromHCL2(v cty.Value) interface{} {
 		it := v.ElementIterator()
 		for it.Next() {
 			ek, ev := it.Element()
-			l[ek.AsString()] = ConfigValueFromHCL2(ev)
+			cv := ConfigValueFromHCL2(ev)
+			if cv != nil {
+				l[ek.AsString()] = cv
+			}
 		}
 		return l
 	}