Fix: Explicitly return null instead of coercing voids in WP_Navigation_Block_Renderer

[justlevine]

Trac ticket: https://core.trac.wordpress.org/ticket/52217

This PR fixes an issue in WP_Navigation_Block_Renderer where ::get_inner_blocks_from_navigation_post() and block_core_navigation_get_classic_menu_fallback() could potentially return void instead of their documented return types.

The issue is remediated by explicitly returning null (how void gets naturally coerced when assigning to a variable), as the most minimal change to address the issue while still persevering backcompat.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

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[justlevine]

wp-includes/blocks is not controlled by this repo