Change to Settings code results in regression of CRM-19610

In Civi\Core/SettingsBag::setDb is a line of code that checks whether the setting in question has an on_change callback.  It should only run a callback if the old value and new value of the setting aren't both null/zero.

As part of CRM-18231, Monish Deb identified a problem.  The code was treating ANY strings as a zero.  However, in fixing it, they caused the string "0" to be evaluated as "non-zero".

This reintroduced the same symptoms seen in CRM-19610, which was the bug that caused the "don't run a callback if old and new values are null/zero" code to be introduced.

References:

https://chat.civicrm.org/civicrm/pl/brdi48bko3rqxy87eefqcnrrwe

https://civicrm.stackexchange.com/questions/21827/unable-to-update-search-preferences-after-upgrading-to-civicrm-4-7-28