fix: Validate CronWorkflow before creation

[simster7]

Fixes: #2529

[codecov]

Codecov Report

Merging #2532 into master will decrease coverage by <.01%.
The diff coverage is 61.36%.

@@            Coverage Diff             @@
##           master    #2532      +/-   ##
==========================================
- Coverage   11.21%   11.21%   -0.01%     
==========================================
  Files          75       75              
  Lines       31765    31769       +4     
==========================================
  Hits         3564     3564              
- Misses      27721    27723       +2     
- Partials      480      482       +2

Impacted Files	Coverage Δ
server/cronworkflow/cron_workflow_server.go	52.63% <59.09%> (-1.66%)	⬇️
server/workflow/workflow_server.go	35.08% <63.63%> (-0.57%)	⬇️

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