Issue _512: Fix null return value from visitLogicalStatement (#547)

* throw an exception if visitlogicalStatement returns a null value

* Made exception message more detailed

Author: Abigael-JT

src/main/java/com/teragrep/pth10/ast/commands/logicalstatement/LogicalStatementXML.java

@@ -334,6 +334,12 @@ else if (ctx.getChildCount() == 3) {
             rv = new ElementNode(el);
         }
 
+        else {
+            throw new IllegalStateException(
+                    "Unexpected number of children: " + ctx.getChildCount() + " in query: " + ctx.getText()
+            );
+        }
+
         if (rv instanceof SubSearchNode) {
             LOGGER.info("[XML] [LogiStat] Return value was SubsearchNode. Converting to ElementNode!");
             return new ElementNode(((SubSearchNode) rv).asElement(doc));