Fix: trying to add a nil substitution to a personalization throws an error

[traviskroberts]

Fixes

If you create a substitution with a nil value, the codebase throws an error if you try to add that substitution to a personalization.

personalization = SendGrid::Personalization.new
personalization.add_substitution(Sendgrid::Substitution.new(key: '%FIRST_NAME%", value: nil))

=> TypeError (no implicit conversion of nil into Hash)

My change will ignore any nil substitutions.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the master branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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