Correct return type for xmlrpc_getposttitle() and xmlrpc_getpostcategory()

[SergeyBiryukov]

There are a few issues in xmlrpc_getposttitle() and xmlrpc_getpostcategory():

• xmlrpc_getposttitle() uses the $post_default_title global as a fallback title, however the global is only set in xmlrpc.php and not in other contexts where the function might be called, e.g. wp-mail.php. In those cases, the function returns null instead of a string, causing issues like #49853.
• xmlrpc_getpostcategory() uses the $post_default_category global, however it was removed in [2680]. Because of that, the function can also return null.
• xmlrpc_getpostcategory() documentation suggests that it returns either a string or an array of strings, however that is incorrect. It's supposed to be used for the post_category parameter of wp_insert_post(), which receives an array of numeric category IDs.

Let's bring some consistency and unit tests here.

[SergeyBiryukov]

Related: #37096