Always pass an initialized set of URls.

Change-Id: Ica072643d407875aa184ed134461252e6bcc93e4
diff --git a/user/src/com/google/gwt/resources/converter/Css2Gss.java b/user/src/com/google/gwt/resources/converter/Css2Gss.java
index ebe6b11..1b43a8a 100644
--- a/user/src/com/google/gwt/resources/converter/Css2Gss.java
+++ b/user/src/com/google/gwt/resources/converter/Css2Gss.java
@@ -272,7 +272,7 @@
 
     boolean recurse;
     boolean singleFile;
-    ImmutableSet<URL> scopeFiles;
+    ImmutableSet<URL> scopeFiles = ImmutableSet.of();
     File resource;
     Set<String> simpleBooleanConditions;