Do not explicitly return TRUE in the FacetapiDependencyFacet::execute() method

This isn't really a bug per-se, however the FacetapiDependency::execute() method employs the hook_node_access() approach where a plugin can either actively satisfy (TRUE), actively reject (FALSE), or ignore (NULL) to let other dependency plugins take action. Instead of returning TRUE, this module should return NULL so it works nicely with other dependency plugins.