Always set base URI path

A SoupURI is considered invalid if the path is NULL. Set the path of the
base URI to an empty string, instead.

This fixes a bunch of these warnings:

    (rhythmbox:32182): libsoup-WARNING **: (soup-uri.c:248):soup_uri_new_with_base: runtime check failed: (SOUP_URI_IS_VALID (base))

libdmapsharing/dmap-connection.c

@@ -1226,6 +1226,7 @@ dmap_connection_setup (DMAPConnection * connection)
 			   connection->priv->host);
 	soup_uri_set_port (connection->priv->base_uri,
 			   connection->priv->port);
+	soup_uri_set_path (connection->priv->base_uri, "");
 }
 
 // FIXME: it would be nice if this mirrored the use of DMAPMdnsBrowser. That is, connect callback handler to a signal.