Checking method availability before calling Log::flushSharedContext()

[ajaxray]

There are some edge cases where $app['log'] may not contain flushSharedContext() method.
In the next statement, in the case of withoutContext(), it's checking for method existence. Similarly, I think i'd be nice to check method existence before calling this function too.

Background

I was trying to use Queue in a Laravel Zero app without installing log component. And was getting this error:

Call to undefined method Psr\Log\NullLogger::flushSharedContext()
at vendor/illuminate/queue/QueueServiceProvider.php:201

Then I checked and found that, the Psr\Log\NullLogger actually don't have this method.

[github-actions]

Thank you for your pull request. However, you have submitted this PR on the Illuminate organization which is a read-only sub split of laravel/framework. Please submit your PR on the https://github.com/laravel/framework repository.

Thanks!