Apollo boards - Fix weight update on move issue

What does this MR do and why?

Fix weight update when moving issue without weight between lists

Behind :apollo_boards feature flag

Issue #428405 (closed)

Screenshots or screen recordings

Before	After
Board_8__3_	Board_8__2_

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:apollo_boards)

2. Visit a board

3. Move an issue without weight between lists and verify that weight doesn't change in list header

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.