[gnome-shell] Fix critical when setting a NULL label

[Giovanni Campagna <gcampagna src gnome org>]

commit e01baf2a2542e10807a724621098b010a0c4a799
Author: Giovanni Campagna <gcampagna src gnome org>
Date:   Mon Aug 1 16:14:23 2011 +0200

    Fix critical when setting a NULL label
    
    st_label_set_text() does not accept NULL, and emits a g_critical
    in that case.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=654638

 js/ui/status/network.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/status/network.js b/js/ui/status/network.js
index a4372b5..a4081ff 100644
--- a/js/ui/status/network.js
+++ b/js/ui/status/network.js
@@ -674,7 +674,7 @@ NMDevice.prototype = {
         let dev_product = this.device.get_product();
         let dev_vendor = this.device.get_vendor();
         if (!dev_product || !dev_vendor)
-	    return null;
+	    return '';
 
         let product = Util.fixupPCIDescription(dev_product);
         let vendor = Util.fixupPCIDescription(dev_vendor);