Allow for zero number of dpi values to be set

At least in the DBus API we can't just say "doesn't exist", so let's allow for
a zero-sized list.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/dbus.rst

@@ -278,6 +278,9 @@ org.freedesktop.ratbag1.Resolution
         the :js:attr:`Resolution` property. This list is always sorted
         ascending, the lowest resolution is the first item in the list.
 
+        This list may be empty if the device does not support reading and/or
+        writing to resolutions.
+
     .. js:attribute:: ReportRate
 
         :type: u

src/libratbag.c

@@ -1131,6 +1131,9 @@ ratbag_resolution_get_dpi_list(struct ratbag_resolution *resolution,
 
 	assert(nres > 0);
 
+	if (resolution->ndpis == 0)
+		return 0;
+
 	memcpy(resolutions, resolution->dpis,
 	       sizeof(unsigned int) * min(nres, resolution->ndpis));