chore: Deduplicate typechecks in setters

[jstevans]

Fixes #553

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• 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 in line documentation to the code I modified

Short description of what this PR does:

• Refactor undefined checks, type checks, etc. into deduplicated functions
• This actually only reduces LOC by 12, but this can be pretty easily moved to a shared file that can be used in other classes.

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[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

@jstevans

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[ ] Sign the CLA before November 3rd
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[jstevans]

@mbernier @thinkingserious Thanks!