Add null check prior to using object.

[nigelgbanks]

What does this Pull Request do?

Checks if an entity is null before de-referencing it.

What's new?

Prevents the following error:

Error: Call to a member function getFileUri() on null in islandora_text_extraction_media_presave() (line 40 of /var/www/drupal/web/modules/contrib/islandora/modules/islandora_text_extraction/islandora_text_extraction.module

How should this be tested?

Small change doesn't really require testing.

Interested parties

@Islandora/8-x-committers

[ajstanley]

Maybe throw something out to the logger if either $file or $file_id is NULL?

[jordandukart]

@nigelgbanks you have re-producible conditions for this?

[nigelgbanks]

Since it takes ages to use the Drupal migrate module, I've got it setup up such that I can run migrations in parallel (Based of this fellows work https://www.drupal.org/project/migrate_tools/issues/3114358). When I create media entities for the first time they may reference files which not have been imported yet. So its a bit of odd use case in that it's not going through the forms to create the entity. That being said there isn't a reason to allow for methods to be called on null values.

[jordandukart]

You able to address #791 (comment) @nigelgbanks?

[nigelgbanks]

Oh sorry I kinda forgot about this, I've just patched locally. I don't see the point in logging in my use case as it's intentionally empty and whatever I can do to speed up these slow migrations I'm keen on. So I'd like to avoid the database update query for logging if possible, as large migrations are painfully slow. That being said if it's gonna block the pull request I'll add some logs, what do you say @ajstanley?

[dannylamb]

...and approved before seeing comments above. An info log statement would be appropriate. Other than that, changes are good and straightforward.

[nigelgbanks]

Another rebase to please the Travis god of CI