skip empty networks in plugin install

Signed-off-by: Victor Vieux <vieux@docker.com>
(cherry picked from commit 04e35a01fcc7292272d688599ed23d829b85f46b)
Signed-off-by: Victor Vieux <vieux@docker.com>

plugin/backend_linux.go

@@ -125,7 +125,7 @@ func computePrivileges(pd distribution.PullData) (types.PluginPrivileges, error)
 	}
 
 	var privileges types.PluginPrivileges
-	if c.Network.Type != "null" && c.Network.Type != "bridge" {
+	if c.Network.Type != "null" && c.Network.Type != "bridge" && c.Network.Type != "" {
 		privileges = append(privileges, types.PluginPrivilege{
 			Name:        "network",
 			Description: "permissions to access a network",