MDL-69477 install: Ensure CFG variables are set before using them

MDL-69477 install: Ensure CFG variables are set before using them

On installation (or also phpunit/behat unit) some CFG variables
were being used (on setting validation) before being set.

So this commit just verifies they are set before using them. Note that,
strictly speaking, only one of them ($CFG->searchenginequeryonly)
required the extra check, but I think it's better to apply it to all
them, as a reference and in case validations are run in any other order.