lib: return empty WpProperties if the info struct has no properties on SM objects

lib/wp/endpoint-link.c

@@ -82,7 +82,8 @@ wp_endpoint_link_get_properties (WpProxy * proxy)
   WpEndpointLink *self = WP_ENDPOINT_LINK (proxy);
   WpEndpointLinkPrivate *priv = wp_endpoint_link_get_instance_private (self);
 
-  return wp_properties_ref (priv->properties);
+  return priv->properties ?
+      wp_properties_ref (priv->properties) : wp_properties_new_empty ();
 }
 
 static struct spa_param_info *

lib/wp/endpoint-stream.c

@@ -80,7 +80,8 @@ wp_endpoint_stream_get_properties (WpProxy * proxy)
   WpEndpointStream *self = WP_ENDPOINT_STREAM (proxy);
   WpEndpointStreamPrivate *priv = wp_endpoint_stream_get_instance_private (self);
 
-  return wp_properties_ref (priv->properties);
+  return priv->properties ?
+      wp_properties_ref (priv->properties) : wp_properties_new_empty ();
 }
 
 static struct spa_param_info *

lib/wp/endpoint.c

@@ -173,7 +173,8 @@ wp_endpoint_get_properties (WpProxy * proxy)
   WpEndpoint *self = WP_ENDPOINT (proxy);
   WpEndpointPrivate *priv = wp_endpoint_get_instance_private (self);
 
-  return wp_properties_ref (priv->properties);
+  return priv->properties ?
+      wp_properties_ref (priv->properties) : wp_properties_new_empty ();
 }
 
 static struct spa_param_info *

lib/wp/session.c

@@ -193,7 +193,8 @@ wp_session_get_properties (WpProxy * proxy)
   WpSession *self = WP_SESSION (proxy);
   WpSessionPrivate *priv = wp_session_get_instance_private (self);
 
-  return wp_properties_ref (priv->properties);
+  return priv->properties ?
+      wp_properties_ref (priv->properties) : wp_properties_new_empty ();
 }
 
 static struct spa_param_info *