Fixes searching items with fulltext

When using fulltext only search, then only the index must be searched.
This wasn't working anymore, because the routes added a query to
always select valid items (those not being processed). But this lead
to the downstream code to always consult the database, too. Since the
routes are using a "simple-search" interface, this is now adding the
valid-state condition if applicable. There are still more low-level
interfaces that can be used when searching should be done differently.

Closes: #823

modules/common/src/main/scala/docspell/common/ItemQueryString.scala

@@ -1,6 +1,9 @@
 package docspell.common
 
-case class ItemQueryString(query: String)
+final case class ItemQueryString(query: String) {
+  def isEmpty: Boolean =
+    query.isEmpty
+}
 
 object ItemQueryString {