return true in isHtaccessWorking when on CLI and no overwrite.cli.url has been configured

fixes cli installation when running travis tests, otherwise an exception like this is thrown
```
#0 /home/jfd/Repositories/oc/core/lib/private/util.php(1161): OC\HTTPHelper-&gt;getUrlContent(&#039;/data/htaccesst...&#039;)
#1 /home/jfd/Repositories/oc/core/lib/private/util.php(1014): OC_Util::getUrlContent(&#039;/data/htaccesst...&#039;)
#2 /home/jfd/Repositories/oc/core/core/setup/controller.php(128): OC_Util::isHtaccessWorking()
#3 /home/jfd/Repositories/oc/core/core/setup/controller.php(15): OC\Core\Setup\Controller-&gt;getSystemInfo()
#4 /home/jfd/Repositories/oc/core/lib/base.php(711): OC\Core\Setup\Controller-&gt;run(Array)
#5 /home/jfd/Repositories/oc/core/index.php(28): OC::handleRequest()
#6 {main}
```

see eg. https://travis-ci.org/owncloud/activity/jobs/37903932#L190

the `overwrite.cli.url` param is only configured after the check is made during installation.

cc @DeepDiver1975