checker: fix for_in_index type error

vlib/v/checker/checker.v

@@ -1663,9 +1663,8 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 					scope.update_var_type(it.key_var, key_type)
 				}
 				value_type := c.table.value_type(typ)
-				if value_type == table.void_type {
-					typ_sym := c.table.get_type_symbol(typ)
-					c.error('for in: cannot index `$typ_sym.name`', it.cond.position())
+				if value_type == table.void_type || typ.has_flag(.optional) {
+					c.error('for in: cannot index `${c.table.type_to_str(typ)}`', it.cond.position())
 				}
 				it.cond_type = typ
 				it.kind = sym.kind

vlib/v/checker/tests/for_in_index_optional.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/for_in_index_optional.v:3:18: error: for in: cannot index `?[]string`
+    1 | import os
+    2 | fn main() {
+    3 |   for file in os.ls('.') {
+      |                  ~~~~~~~
+    4 |     println(file)
+    5 |   }

vlib/v/checker/tests/for_in_index_optional.vv

@@ -0,0 +1,6 @@
+import os
+fn main() {
+  for file in os.ls('.') {
+    println(file)
+  }
+}

vlib/v/checker/tests/for_in_index_type.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/for-in-index-type.v:2:11: error: for in: cannot index `any_int` 
+vlib/v/checker/tests/for_in_index_type.v:2:11: error: for in: cannot index `any_int` 
     1 | fn main() {
     2 |     for a in 52 {
       |              ~~