Avoid rewriting quoted SQL

If your database contains text fields which contain

```
'0000-00-00 00:00:00'
```

then the existing search/replace with NULL would create syntax errors by accidentally closing the quotes.

```
'\'0000-00-00 00:00:00\'' # one string, from MySQL
'''0000-00-00 00:00:00''' # one string, after replacing \' with ''
''NULL''                  # syntax error: string NULL string
```

This change should avoid this problem.