docs: rename ql-training-rst > ql-training

docs/language/ql-training/query-examples/cpp/empty-if-cpp.ql

@@ -0,0 +1,7 @@
+import cpp  
+   
+from IfStmt ifstmt, Block block
+where
+  block = ifstmt.getThen() and
+  block.isEmpty()
+select ifstmt, "This if-statement is redundant."