patch 9.0.1745: Missing test coverage for blockwise Visual highlight

Problem:  Missing test coverage for blockwise Visual highlight with
          virtual that starts with a double-width char.
Solution: Add a new virtual text to the test. Some other small fixes.

closes: #12835

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/dict.c

@@ -1497,7 +1497,7 @@ dict2list(typval_T *argvars, typval_T *rettv, dict2list_T what)
 
     d = argvars[0].vval.v_dict;
     if (d == NULL)
-	// empty dict behaves like an empty dict
+	// NULL dict behaves like an empty dict
 	return;
 
     todo = (int)d->dv_hashtab.ht_used;