fix loo_model_weights naming for list of loo objects

[jgabry]

fixes #165 by getting model names for loo_model_weights even if list of loo objects is only partially named

@avehtari Does this behave as expected now?

[codecov-commenter]

Codecov Report

Merging #217 (39cafc0) into master (1832085) will increase coverage by 0.04%.
The diff coverage is 88.88%.

❗ Current head 39cafc0 differs from pull request most recent head 8828a7d. Consider uploading reports for the commit 8828a7d to get more accurate results

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   93.05%   93.09%   +0.04%     
==========================================
  Files          30       30              
  Lines        2766     2768       +2     
==========================================
+ Hits         2574     2577       +3     
+ Misses        192      191       -1     

Impacted Files	Coverage Δ
R/loo_model_weights.R	99.27% <87.50%> (+0.01%)	⬆️
R/loo_compare.R	95.83% <100.00%> (+1.04%)	⬆️

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[avehtari]

Tested. Works now as expected.