Themes: Prevent null deprecation in previous_posts().

[dan-zakirov]

This adds a null guard to previous_posts(), matching the existing pattern used in next_posts()

Without this check, calling previous_posts() on a singular view can pass null to esc_url(), which in turn triggers an ltrim() deprecation notice on PHP 8.1 and newer

Props @dd32
Fixes #64864

Trac ticket: https://core.trac.wordpress.org/ticket/64864

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