CAUSEWAY-3831: fixes dynamic visibility of search-bar based

- by getting the number of table rows  directly from the underlying
model

viewers/wicket/ui/src/main/java/org/apache/causeway/viewer/wicket/ui/components/table/DataTableWithPagesAndFilter.java

@@ -105,6 +105,14 @@ protected void onConfigure() {
         honorPageNumberHint();
     }
 
+    /**
+     * Gets the number of table rows (unfiltered) directly from the underlying model.
+     * While {@link #getRowCount()} might return zero, when the component was no yet populated.
+     */
+    public int elementCount() {
+        return _TableUtils.interactive(this).dataElementsObservable().getValue().size();
+    }
+
     // -- SELECTABLE
 
     /**

viewers/wicket/ui/src/main/java/org/apache/causeway/viewer/wicket/ui/components/table/filter/FilterToolbar.java

@@ -55,7 +55,7 @@ public FilterToolbar(final String id, final DataTableWithPagesAndFilter<?, ?> ta
     protected void onConfigure() {
         super.onConfigure();
         buildGui();
-        setVisible(getTable().getRowCount() > 1);
+        setVisible(table.elementCount() > 1);
     }
 
     // -- HELPER