[banshee/gtk3] build: use '' as empty string in GSettingsSchemaExtractor

[AndrÃs Aragoneses <aaragoneses src gnome org>]

commit f6bd7349e208407042bd8f6a6e7ec30cd68d6f47
Author: Andres G. Aragoneses <knocte gmail com>
Date:   Wed Dec 19 01:02:37 2012 +0000

    build: use '' as empty string in GSettingsSchemaExtractor
    
    An empty string wasn't valid simply as <default></default>, but we needed
    quotes on it like other strings. Otherwise this would fail at build time:
    
    org.gnome.banshee.gschema.xml: Error on line 194 char 1: 0:expected value.  --strict was specified; exiting.
    make[2]: *** [org.gnome.banshee.gschema.valid] Error 1
    make[1]: *** [all-recursive] Error 1
    make: *** [all] Error 2

 build/GSettingsSchemaExtractor.cs      |    3 ++-
 build/GSettingsSchemaExtractorTests.cs |    2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/build/GSettingsSchemaExtractor.cs b/build/GSettingsSchemaExtractor.cs
index c5fd156..6cc4da0 100644
--- a/build/GSettingsSchemaExtractor.cs
+++ b/build/GSettingsSchemaExtractor.cs
@@ -132,7 +132,8 @@ public class GSettingsSchemaExtractorProgram
             gctype = "d";
         } else if (type == typeof (string)) {
             gctype = "s";
-            return o == null ? null : "'" + o.ToString () + "'";
+            string value = o == null ? String.Empty : o.ToString ();
+            return String.Format ("'{0}'", value);
         } else {
             throw new Exception("Unsupported type '" + type + "'");
         }
diff --git a/build/GSettingsSchemaExtractorTests.cs b/build/GSettingsSchemaExtractorTests.cs
index ccd12d0..78e4052 100644
--- a/build/GSettingsSchemaExtractorTests.cs
+++ b/build/GSettingsSchemaExtractorTests.cs
@@ -229,7 +229,7 @@ namespace GSettingsSchemaExtractor
 <schemalist>
   <schema id=""org.gnome.banshee.plugins.audioscrobbler"" path=""/apps/banshee/plugins/audioscrobbler/"" gettext-domain=""banshee"">
     <key name=""api-url"" type=""s"">
-      <default></default>
+      <default>''</default>
       <summary>AudioScrobbler API URL</summary>
       <description>URL for the AudioScrobbler API (supports turtle.libre.fm, for instance)</description>
     </key>