Forbid empty sequences of imperative statements

[anton-trunov]

Because when auditing contracts it can sometimes be hard to tell if the contract author just forgot to put a piece of code in some of match-statements and this change would make this explicit.

For example, the following

match is_valid with 
| True => 
| False =>
    error = FooError;
    Throw error
end

would be replaced with

match is_valid with 
| True =>
    do_nothing   (* or `nop` or whatever syntax we choose *)
| False =>
    error = FooError;
    Throw error
end