Harden Attr/Relationship attributes against invalid input

[bkoelman]

This PR hardens Attr/Relationship attributes against invalid input.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Merging #1268 (149c4f6) into master (5301d04) will increase coverage by 0.15%.
The diff coverage is 100.00%.

❗ Current head 149c4f6 differs from pull request most recent head 338c63e. Consider uploading reports for the commit 338c63e to get more accurate results

@@            Coverage Diff             @@
##           master    #1268      +/-   ##
==========================================
+ Coverage   92.83%   92.98%   +0.15%     
==========================================
  Files         255      255              
  Lines        8221     8242      +21     
==========================================
+ Hits         7632     7664      +32     
+ Misses        589      578      -11     

Impacted Files	Coverage Δ
...Annotations/Resources/Annotations/AttrAttribute.cs	90.00% <ø> (ø)
...ons/Resources/Annotations/RelationshipAttribute.cs	88.88% <ø> (ø)
...otations/Resources/Annotations/HasManyAttribute.cs	100.00% <100.00%> (ø)
...notations/Resources/Annotations/HasOneAttribute.cs	100.00% <100.00%> (ø)
...ns/Resources/Annotations/ResourceFieldAttribute.cs	93.33% <100.00%> (+28.33%)	⬆️
src/JsonApiDotNetCore/Queries/FieldSelectors.cs	100.00% <100.00%> (ø)
...otNetCore/Queries/Internal/Parsing/FilterParser.cs	98.40% <100.00%> (ø)
.../Internal/QueryableBuilding/SelectClauseBuilder.cs	99.13% <100.00%> (ø)
...Core/Repositories/EntityFrameworkCoreRepository.cs	96.62% <100.00%> (+0.01%)	⬆️