allow inferred error sets to be empty

[andrewrk]

test case:

fn foo() !void {}

test "" {
    foo() catch |err| switch (err) {};
}

status quo:

$ ./zig test test2.zig 
./test2.zig:1:11: error: function with inferred error set must return at least one possible error
fn foo() !void {}
          ^
./test2.zig:4:11: note: referenced here
    foo() catch |err| switch (err) {};
          ^

accepted proposal:

$ ./zig test test2.zig 
All 1 tests passed.

It would be equivalent to this code:

fn foo() error{}!void {}

test "" {
    foo() catch |err| switch (err) {};
}