[DATABASE] Disable 'NULL' strings evaluation as SQL NULLs

Use $object->sqlValue('NULL') (identical to DataObject_Cast'ing) instead and
fix related issues like (email|sms)settings considering these NULLs as a
false positive for the E-Mail address still being set when it's been removed.

There could also be security implications to the now-disabled approach of
considering 'NULL' strings as SQL NULLs.

lib/util/default.php

@@ -74,6 +74,7 @@ $default =
                 'mirror' => null,
                 'utf8' => true,
                 'db_driver' => 'DB', # XXX: JanRain libs only work with DB
+                'disable_null_strings' => true, // 'NULL' can be harmful
                 'quote_identifiers' => true,
                 'type' => 'mysql',
                 'schemacheck' => 'runtime', // 'runtime' or 'script'