runfix: identify mls capable client

[PatrykBuniX]

Description

Fixes the way we identify a client being MLS capable. We were mistakenly assuming that client with mls_public_keys field is an mls client (even if this field contained empty object, see wireapp/wire-web-packages#5516). mls_public_keys is empty object by default, we need to check if it really contains any values (Public key is uploaded after mls device is registered).

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;

[codecov]

Codecov Report

Merging #15841 (622104d) into dev (2914fe6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev   #15841   +/-   ##
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  Coverage   44.48%   44.48%           
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  Files         673      673           
  Lines       22715    22717    +2     
  Branches     5166     5166           
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+ Hits        10104    10106    +2     
  Misses      11323    11323           
  Partials     1288     1288