API regression on return value for missing plugin

[tellyworth]

There's a mismatch between the old API and new when fetching info for a plugin that doesn't exist:

​https://api.wordpress.org/plugins/info/1.0/user-switching-not-a-plugin.json
null

vs

​https://api.wordpress.org/plugins/info/1.0/user-switching-not-a-plugin.json?wp=1
{"error":"Plugin not found.","contributors":[]}

This trips up some clients. See ​https://wordpress.slack.com/archives/meta/p1482353516001438.

[dd32]

In 4566:

Plugin Directory: Handle API failures better, and output 'null' when an individual plugin API call fails, for backwards compatibility.

Fixes #2338

[dd32]

In 4570:

Plugin Directory: Follow up to [4566]; in the event that a falsey value is sent to the client, do not convert it to an object for the PHP serialized output handler.

See #2338