Fix deleting properties of user settings when not given explicitly

The controller can receive an optional subset of the properties of the
user settings; values not given are set to "null" by default. However,
those null values overwrote the previously existing values, so in
practice any value not given was deleted from the user settings. Now
only non null values overwrite the previous values.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>