[sysprof] libsysprof-ui: fix some warnings

[Christian Hergert <chergert src gnome org>]

commit cf1d6a9c0768ef79b8f69f433c9f7a12bddb46ac
Author: Christian Hergert <chergert redhat com>
Date:   Thu May 16 15:55:12 2019 -0700

    libsysprof-ui: fix some warnings

 src/libsysprof-ui/ui/sysprof-profiler-menu-button.ui | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/libsysprof-ui/ui/sysprof-profiler-menu-button.ui 
b/src/libsysprof-ui/ui/sysprof-profiler-menu-button.ui
index 2f6b854..594dc4c 100644
--- a/src/libsysprof-ui/ui/sysprof-profiler-menu-button.ui
+++ b/src/libsysprof-ui/ui/sysprof-profiler-menu-button.ui
@@ -227,8 +227,8 @@
     </columns>
     <data>
       <row>
-        <col id="0"></col>
-        <col id="1"></col>
+        <col id="0">''</col>
+        <col id="1">''</col>
       </row>
     </data>
   </object>