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