Fixed sanitize_term_field missing default value

[dilipbheda]

Trac ticket: https://core.trac.wordpress.org/ticket/63257

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[rollybueno]

Looks good to me 👍

[SirLouen]

Not a big deal, but technically correct.

[mindctrl]

@dilipbheda could you bring your PR up to date with the trunk branch? This will ensure the patch applies cleanly and will also run the test suite again (which failed for some reason, but the logs are no longer available to understand why).

[t-hamano]

could you bring your PR up to date with the trunk branch?

I handled it.

[mindctrl]

Given your question on the Trac ticket, I think we should punt this one and leave room for discussion and discovery.

[t-hamano]

Setting a default value may cause backward compatibility issues. It's probably sufficient to simply update the DocBlock. See #10918