parser: Do now allow the empty symbol declaration
An empty element is allowed in SymbolsBody definition, so the following
keymap is gramatically correct.
```
xkb_keymap {
...
xkb_symbols "sym" {
key <SPC> {, [Space] };
};
};
```
However, the current parser crashes with the keymap due to null pointer
access.
This change fixes it by changing the parser not to allow it.
