[5.x] Prevent the Link Fieldtype's Asset option from erroring when container is empty

[duncanmcclean]

This pull request fixes an issue where the Link Fieldtype would consider the "Asset" link option to be active, even when the container config value is an empty array.

This PR fixes it by using the empty function instead of just checking that it's not null.

Fixes #10268.

[morhi]

Hey @duncanmcclean :) I wonder if this really the right fix? To me it seems that the Dropdown component should not return an empty array, but null, if it is supposed to return one item only. Otherwise other references which use the return value of the Dropdown component might be broken, too, like my Addon :)

Edit: In Statamic 5.5.0 the fieldtype config() doesn't even contain the container property, if there is no asset container set. I think the null comparison is because the default config value is null if unset.

[duncanmcclean]

Closing in favour of #10272.