Fix PHP notice that showed when orderby query parameter was an array

[leonidasmi]

Trac ticket: https://core.trac.wordpress.org/ticket/59494

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Props leonidasmilossis, peterwilsoncc.

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[peterwilsoncc]

Committed in r58379 / https://github.com/WordPress/wordpress-develop/commit/8cd24bc12c7ac3c588ccbf549b1df42884181fe0