[libcalamares] Asking for type of undefined node throws

- Use YAML-CPP API for finding out if a node has a value at all.
 - Asking for Type() of an undefined or NULL node throws an
   exception, so the existing code didn't **actually** catch
   cases where a required setting wasn't set at all.

src/libcalamares/Settings.cpp

@@ -32,7 +32,7 @@
 static bool
 hasValue( const YAML::Node& v )
 {
-    return !( ( v.Type() == YAML::NodeType::Null ) || ( v.Type() == YAML::NodeType::Undefined ) );
+    return v.IsDefined() && !v.IsNull();
 }
 
 /** Helper function to grab a QString out of the config, and to warn if not present. */