We need to expose null literals in parse trees, otherwise, it will not
be possible to syntactically differentiate between some constructs.
Example: currently, these two functions yield the same parse tree:
```
fun f() = null
fun f() { }
```
```
source_file
  function_declaration
    simple_identifier
    function_value_parameters
    function_body
```

With the changes in this PR, the first function will have a
`null_literal` in the tree:
```
source_file
  function_declaration
    simple_identifier
    function_value_parameters
    function_body
      null_literal
```