Can anyone explain what the difference is in these two "empty strings"?

Question

So, I had a table where the default was NULL on an ENUM column. I did some testing, setting some rows to various values, and then updated one of the rows to a value not in the ENUM. I then got this: MariaDB [my_db]> select this_value, count(*) FROM my_table GROUP by this_value;

+--------------------+----------+
| this_value         | count(*) |
+--------------------+----------+
| NULL               |      242 |
|                    |        1 |
| AA                 |        1 |
| AB                 |        1 |
| AC                 |        1 |
| BA                 |        1 |
| BB                 |        1 |
| BC                 |        1 |
+--------------------+----------+
8 rows in set (0.001 sec)

Then, I did a ALTER TABLE my_table MODIFY COLUMN this_value, adding the '' to the list of value options and making the default ''.

Now, this is what you get:

MariaDB [my_db]> select this_value, count(*) FROM my_table GROUP by this_value;
+--------------------+----------+
| this_value         | count(*) |
+--------------------+----------+
|                    |        1 |
|                    |      242 |
| AA                 |        1 |
| AB                 |        1 |
| AC                 |        1 |
| BA                 |        1 |
| BB                 |        1 |
| BC                 |        1 |
+--------------------+----------+
8 rows in set (0.001 sec)

So, what is the difference between the two "empty strings". The whole goal of this was because with a query like this_value NOT LIKE 'A%', we were not getting the NULL values (and we wanted those, since they were "not starting with A" values), and this does resolve to that ("SELECT count(*) FROM my_table WHERE this_value NOT LIKE "A%" returns 246), but, I am just interested in how Mysql is differentiating those two values... doing a CHAR_LENGTH on it shows they are both 0, but obviously they still aren't exactly "equivalent" to mysql.