Update conditional check for settings_fields in settings_section

[alpipego]

When a section did not have fields attached, the loop moved to the next item. By inverting the logic check for fields, this change makes sure the after_section argument gets printed.

Trac ticket: https://core.trac.wordpress.org/ticket/62746

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Props alpipego.

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[github-actions]

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[SergeyBiryukov]

Thanks for the PR! Merged in r59564.