add tests when using .len on other types

cmd/tools/vvet/tests/empty_string.vv

@@ -9,4 +9,13 @@ fn main() {
 
 	if foo.len < 1 || bar.len > 0 || (foo.len == 0 && bar.len == 0) {
 	}
+
+	// Should not notify when `.len` is used with other types.
+	baz := ['baz']
+	_ := baz.len == 0
+
+	foobar := {
+		'foo': 'bar'
+	}
+	_ := foobar.len < 1
 }