Empty string column in caml query

spSiteDataQuery = new SPSiteDataQuery();
                   spSiteDataQuery.Query = "<Where><Neq><FieldRef Name=\"ContentType\" /><Value Type=\"Text\">" + "</Value></Neq></Where>";
                   spSiteDataQuery.ViewFields = "<FieldRef Name=\"Title\"/>" + "<FieldRef Name=\"FileRef\" />" + "<FieldRef Name=\"Modified\"/>" + "<FieldRef Name=\"Created\"/>";
                   spSiteDataQuery.Lists = "<Lists ServerTemplate=\"850\"/>";
                   spSiteDataQuery.Webs = "<Webs Scope=\"Recursive\" />";

                   dtPages = spWeb.GetSiteData(spSiteDataQuery);

<value type ...></value>;

<IsNull></IsNull>;