Braces not allowed for reactors without parameters

[lhstrh]

Our grammar does not allow:

reactor() {
  ...
}

but there is no clear reason for this restriction (and the resulting error message is not particularly clear).
We can allow empty braces this by writing (('(' parameters+=Parameter (',' parameters+=Parameter)* ')') | '(' ')')?
instead of ('(' parameters+=Parameter (',' parameters+=Parameter)* ')')?.