Validate weights fix

[n-kall]

Summary

Fixes #347 by not allowing missing values in weights.

Copyright and Licensing

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license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[paul-buerkner]

thanks!

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 00bf4da is merged into master:

•   :ballot_box_with_check:as_draws_array: 142ms -> 142ms [-1.34%, +1.36%]
•   :ballot_box_with_check:as_draws_df: 71.8ms -> 71.4ms [-1.29%, +0.33%]
•   :ballot_box_with_check:as_draws_list: 173ms -> 172ms [-1.28%, +0.61%]
• ❗🐌as_draws_matrix: 62.9ms -> 65.1ms [+2.34%, +4.69%]
•   :ballot_box_with_check:as_draws_rvars: 85ms -> 84.6ms [-1.6%, +0.84%]
•   :ballot_box_with_check:summarise_draws_100_variables: 712ms -> 712ms [-0.33%, +0.42%]
•   :ballot_box_with_check:summarise_draws_10_variables: 116ms -> 116ms [-0.91%, +1.04%]
  Further explanation regarding interpretation and methodology can be found in the documentation.