Return empty layout model is layout delegate is empty.

[chupakabr]

Fixes an issue of returning layout attributes for old layout.

[codecov-io]

Codecov Report

Merging #304 into master will decrease coverage by 0.19%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #304     +/-   ##
=========================================
- Coverage    64.4%   64.21%   -0.2%     
=========================================
  Files          63       63             
  Lines        3523     3532      +9     
=========================================
- Hits         2269     2268      -1     
- Misses       1254     1264     +10

Impacted Files	Coverage Δ
...oller/Collaborators/ChatCollectionViewLayout.swift	80% <0%> (-16.08%)	❌

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