[11.x] Fix crash when configuration directory is non-existing

[buismaarten]

Since the Laravel configuration files are no longer necessarily needed in the skeleton, some may want to delete the configuration directory entirely.

When the configuration directory is omitted, Laravel crashes using the following error:

The error occurs because $configPath = realpath($app->configPath()); returnsfalse, this triggers a Exception because false is not a directory that can be found.

This PR solves this problem where the configuration directory can be omitted without crash.