[gnome-software] shell-extensions: Fix a warning on a NULL UUID

[Richard Hughes <rhughes src gnome org>]

commit 0faa8871ba4506b0d7ac148127d73909645f07eb
Author: Philip Withnall <withnall endlessm com>
Date:   Mon Jul 29 17:59:50 2019 +0100

    shell-extensions: Fix a warning on a NULL UUID
    
    This fixes the warning:
    ```
    failed to process 
components/component/custom/value[@key='shell-extensions::uuid'][text()='(null)']/../..: failed to parse 
part: cannot parse text or number `null`
    ```
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 plugins/shell-extensions/gs-plugin-shell-extensions.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/plugins/shell-extensions/gs-plugin-shell-extensions.c 
b/plugins/shell-extensions/gs-plugin-shell-extensions.c
index 77dcb0c8..2873ec8a 100644
--- a/plugins/shell-extensions/gs-plugin-shell-extensions.c
+++ b/plugins/shell-extensions/gs-plugin-shell-extensions.c
@@ -460,6 +460,9 @@ gs_plugin_refine_app (GsPlugin *plugin,
        locker = g_rw_lock_reader_locker_new (&priv->silo_lock);
 
        /* find the component using the UUID */
+       if (uuid == NULL)
+               return TRUE;
+
        xpath = g_strdup_printf ("components/component/custom/"
                                 "value[@key='shell-extensions::uuid'][text()='%s']/../..",
                                 uuid);