Plugins: After [41608], make the markup in...

Plugins: After [41608], make the markup in `WP_Plugin_Install_List_Table::no_items()` a bit more readable.

See #37430.
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git-svn-id: https://core.svn.wordpress.org/trunk@41749 1a063a9b-81f0-0310-95a4-ce76da25c4cd