Check if key is defined befory loading the entity, fix array_flip warning

[idflood]

On a view with multiple selective filters I had this warning showing up (only for anonymous users):

Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 227 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).

Drupal\Core\Entity\EntityStorageBase->loadMultiple(Array) (Line: 212)
Drupal\Core\Entity\EntityStorageBase->load() (Line: 461)
Drupal\views_selective_filters\Plugin\views\filter\Selective->getOids() (Line: 86)

It appears that the $key was sometimes false.

[joelpittet]

Why not add the $key empty check to the condition above?

[idflood]

@joelpittet good point, I moved the $key condition above as you suggested.

[bmcclure]

Thanks for this PR! I will test out the change and get it merged ASAP.

[jvandenrym]

I get this warning. Will any of the above fix this warning. Tried the above but to no avail.

Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Drupal\Core\Entity\EntityStorageBase->loadMultiple(Array) (Line: 249)
Drupal\Core\Entity\EntityStorageBase->load() (Line: 460)
Drupal\views_selective_filters\Plugin\views\filter\Selective->getOids() (Line: 86)
Drupal\views_selective_filters\Plugin\views\filter\Selective->getValueOptions() (Line: 114)
Drupal\views_selective_filters\Plugin\views\filter\Selective->valueForm(Array, Object) (Line: 861)
Drupal\views\Plugin\views\filter\FilterPluginBase->buildExposedForm(Array, Object) (Line: 96)`
...
   ...

[idflood]

@jvandenrym yes it should be fixed on this is merged, at least your error message seems to be exactly what I had before.

[KaseyMK]

This change fixes the array_flip error on my use-case, too. Can it be merged? Thank you!