tools.vet: add notice for empty strings conditions

[ttytm]

After getting a response from Alex that it's fine to add the suggestion made in #21300 this should be the frist extension after #21417.

fn main() {
	foo := 'foo'
	foo.len == 0
	foo.len != 0
	foo.len < 1
	foo.len > 0
}

will result in:

cmd/tools/vvet/tests/empty_string.vv:3: notice: Use `foo == ''` instead of `foo.len == 0`
cmd/tools/vvet/tests/empty_string.vv:4: notice: Use `foo != ''` instead of `foo.len != 0`
cmd/tools/vvet/tests/empty_string.vv:5: notice: Use `foo == ''` instead of `foo.len < 1`
cmd/tools/vvet/tests/empty_string.vv:6: notice: Use `foo != ''` instead of `foo.len > 0`

---

To keep this as a ref.: as mentioned here(#21523 (comment)), adding this check for globally scoped symbols is still a TODO.

[spytheman]

The Vinix failure is not related, and already fixed on master.

[spytheman]

Excellent work.