lxc: allow empty path in URI for historical compatibility

The use of 'lxc://' was mistakenly broken in:

  commit 4c8574c8
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Mar 28 12:49:29 2018 +0100

    driver: ensure NULL URI isn't passed to drivers with whitelisted URIs

Allow it again for historical compatibility.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/lxc/lxc_driver.c

@@ -140,7 +140,8 @@ static virDrvOpenStatus lxcConnectOpen(virConnectPtr conn,
     virCheckFlags(VIR_CONNECT_RO, VIR_DRV_OPEN_ERROR);
 
     /* If path isn't '/' then they typoed, tell them correct path */
-    if (STRNEQ(conn->uri->path, "/") &&
+    if (STRNEQ(conn->uri->path, "") &&
+        STRNEQ(conn->uri->path, "/") &&
         STRNEQ(conn->uri->path, "/system")) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("Unexpected LXC URI path '%s', try lxc:///system"),