[libcalamares] Complain if key isn't set

- Previous check would also fail when the setting is false,
   not just when the key is missing.

src/libcalamares/Settings.cpp

@@ -29,13 +29,19 @@
 
 #include <yaml-cpp/yaml.h>
 
+static bool
+hasValue( const YAML::Node& v )
+{
+    return !( ( v.Type() == YAML::NodeType::Null ) || ( v.Type() == YAML::NodeType::Undefined ) );
+}
 
 /** Helper function to grab a QString out of the config, and to warn if not present. */
 static QString
 requireString( const YAML::Node& config, const char* key )
 {
-    if ( config[ key ] )
-        return QString::fromStdString( config[ key ].as< std::string >() );
+    auto v = config[ key ];
+    if ( hasValue(v) )
+        return QString::fromStdString( v.as< std::string >() );
     else
     {
         cWarning() << "Required settings.conf key" << key << "is missing.";
@@ -47,8 +53,9 @@ requireString( const YAML::Node& config, const char* key )
 static bool
 requireBool( const YAML::Node& config, const char* key, bool d )
 {
-    if ( config[ key ] )
-        return config[ key ].as< bool >();
+    auto v = config[ key ];
+    if ( hasValue(v) )
+        return v.as< bool >();
     else
     {
         cWarning() << "Required settings.conf key" << key << "is missing.";