REST API: Fix PHP warnings when `get_theme_support( 'post-formats' )` is not an array.

If `add_theme_support( 'post-formats' )` is called with no additional
arguments, then `get_theme_support( 'post-formats' )` returns `true` rather
than an array of supported formats.  Avoid generating PHP warnings in this
situation.

Props dreamon11, ChopinBach.
Fixes #39293.

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