added how to create empty vectors and matrices

Author: avehtari

src/reference-manual/expressions.qmd

@@ -301,6 +301,19 @@ The empty expression `[ ]` is ambiguous and therefore is not
 allowed and similarly expressions such as `[ [ ] ]` or
 `[ [ ], [ ] ]` are not allowed.
 
+#### Empty vectors and matrices {-}
+
+If needed, it is possible to create an empty vector with
+```stan
+rep_vector(e, 0)
+```
+where the first expression `e` needs to scalar of type `real`.
+
+If needed, it is possible to create an empty matrix with
+```stan
+rep_matrix(e, 0, 0)
+```
+where the first expression `e` needs to scalar of type `real`.
 
 ### Array expressions {-}