make filter boards an empty string by default.

fixes a crash when the boards were not changed at all when adding
a filter, and as such it was never set to an empty string by the
saveBoardsToFilter method.

Clover/app/src/main/java/org/floens/chan/core/model/orm/Filter.java

@@ -41,7 +41,7 @@ public class Filter {
     public boolean allBoards = true;
 
     @DatabaseField(canBeNull = false)
-    public String boards;
+    public String boards = "";
 
     @DatabaseField(canBeNull = false)
     public int action;