Fix Conjunction.addOperator to do nothing if term is empty

prevent to result in query string with repeated logical operator
like "field:term AND AND field:term"
possibliy causing out of mem in postprocessing_doublecontent

source/net/yacy/cora/federate/solr/logic/AbstractOperations.java

@@ -20,10 +20,17 @@ public abstract class AbstractOperations extends AbstractTerm implements Operati
         return h;
     }
 
+    /**
+     * Add the term with the defined logical operator.
+     * If length of term is empty nothing is changed
+     * @param term
+     */
     @Override
     public void addOperand(Term term) {
+        if (term.toString().isEmpty()) {
             this.terms.add(term);
         }
+    }
     
     /**
      * As a Operations object is a collection of Terms, we must be able to show them