[cbench] Fix return type of Benchmark.validate().

[ChrisCummins]

Fixes a regression in cBench validators. The validator should return an iterable of ValidationError, not an Optional[ValidationError]. Since I don't have a testcase to cover that, I only discovered the problem by breaking a program.

I should follow up with a new testcase.

[codecov-commenter]

Codecov Report

Merging #604 (0c685b4) into development (d5d60b6) will decrease coverage by 0.16%.
The diff coverage is 69.23%.

@@               Coverage Diff               @@
##           development     #604      +/-   ##
===============================================
- Coverage        88.26%   88.10%   -0.17%     
===============================================
  Files              114      114              
  Lines             6708     6715       +7     
===============================================
- Hits              5921     5916       -5     
- Misses             787      799      +12     

Impacted Files	Coverage Δ
compiler_gym/bin/validate.py	87.09% <66.66%> (ø)
compiler_gym/envs/llvm/datasets/cbench.py	79.42% <70.00%> (-1.32%)	⬇️
compiler_gym/views/observation_space_spec.py	85.71% <0.00%> (-3.58%)	⬇️
compiler_gym/views/observation.py	97.29% <0.00%> (-2.71%)	⬇️
compiler_gym/envs/compiler_env.py	91.33% <0.00%> (-1.00%)	⬇️
compiler_gym/envs/gcc/service/gcc_service.py	96.70% <0.00%> (-0.37%)	⬇️

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