py/objdict: Implement bool and len unary ops for dict views.

Currently, dict views (eg `dict.keys()`, `dict.values()`) do not implement
the `bool` or `len` unary operations.  That may seem like a reasonable
omission for MicroPython to keep code size down, but it actually leads to
silently incorrect bool operations, because by default things are true.

Eg we currently have:

    >>> bool(dict().keys())
    True

which is wrong, it should be `False` because the dict is empty.

This commit implements `bool` and `len` unary operations on dict views by
simply delegating to the existing dict unary op function.

Fixes issue #12385.

Signed-off-by: Damien George <damien@micropython.org>

py/objdict.c

@@ -521,7 +521,8 @@ static mp_obj_t dict_view_unary_op(mp_unary_op_t op, mp_obj_t o_in) {
     if (op == MP_UNARY_OP_HASH && o->kind == MP_DICT_VIEW_VALUES) {
         return MP_OBJ_NEW_SMALL_INT((mp_uint_t)o_in);
     }
-    return MP_OBJ_NULL;
+    // delegate all other ops to dict unary op handler
+    return dict_unary_op(op, o->dict);
 }
 
 static mp_obj_t dict_view_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_in) {