Better handling of log file failures (#9539)

Fix issues when APP_LOG is empty, unwritable , or pointed at a directory.
APP_LOG takes precedence over $config['log_file'] now.

DO NOT DELETE THIS TEXT

#### Please note

> Please read this information carefully. You can run `./scripts/pre-commit.php` to check your code before submitting.

- [x] Have you followed our [code guidelines?](http://docs.librenms.org/Developing/Code-Guidelines/)

#### Testers

If you would like to test this pull request then please run: `./scripts/github-apply <pr_id>`, i.e `./scripts/github-apply 5926`
After you are done testing, you can remove the changes with `./scripts/github-remove`.  If there are schema changes, you can ask on discord how to revert.

bootstrap/app.php

@@ -42,8 +42,8 @@ $app->singleton(
 );
 
 $app->configureMonologUsing(function (Monolog\Logger $logger) use ($app) {
-    $path = $app->basePath(config('app.log'));
-    $logger->pushHandler(new \Monolog\Handler\StreamHandler($path));
+    $path = config('app.log') ?: $app->basePath('logs/librenms.log');
+    $logger->pushHandler(new \Monolog\Handler\StreamHandler($path, \Monolog\Logger::toMonologLevel(config('app.log_level', 'debug'))));
 });
 
 /*