[57545] introduced `wp_get_plugin_action_button()`. This function is documented to return a `string`. However, if the user does not have the appropriate capabilities, it returned `void`, which is unexpected.
Resolves the issue by moving the `return $button` to the bottom of the function to ensure it always returns a `string` type. On success, the button's HTML string is returned; else, an empty string is returned.
Unit tests are included.
Follow-up to [57545].
Props costdev, rajinsharwar, hellofromTonya.
Fixes#61400.
Built from https://develop.svn.wordpress.org/trunk@58396
git-svn-id: http://core.svn.wordpress.org/trunk@57845 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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