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>
diff --git a/src/dict.c b/src/dict.c
index ccd0b5d..ce80735 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -1497,7 +1497,7 @@
 
     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;