parser error message for empty variables not helpful

The parser prints out a non-helpful error message when setting an empty variable.

@{foo} =

-> AppArmor parser error, in stdin line 2: syntax error, unexpected $end, expecting TOK_VALUE

@{foo} =
/foo { }

-> AppArmor parser error, in stdin line 2: syntax error, unexpected TOK_ID, expecting TOK_VALUE

A message saying "Variables must at least have one value" would be more helpful.