get_theme_updates() assumes update data is present (regression)

[johnbillion]

Changes made to get_theme_updates() as part of #20103 mean it now assumes that theme update data is present in the 'update_themes' transient.

If theme updates have been disabled then this data is not present and a PHP warning is triggered when you view the WordPress Updates screen.

[johnbillion]

Patch

[nacin]

We should probably double-check other transient calls to ensure this is consistent.

[nacin]

Looks good.

[nacin]

In [20438]:

Check if the response property is set before continuing with the update_themes transient return value. props johnbillion, fixes #20378.