Avoid violation of G-2340 in G-2330

The bad examples of [G-2330](https://trivadis.github.io/plsql-and-sql-coding-guidelines/v4.2/4-language-usage/2-variables-and-types/3-character-data-types/g-2330/#example-bad) the following guidelines should use CHAR SEMANTIC:

```sql
co_null_string constant varchar2(1 char) := '';
```

instead of

```sql
co_null_string constant varchar2(1) := '';
```
