Make Mail helper parameters truly optional

[onecrayon]

Fixes #569

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.
• n/a I have added tests that prove my fix is effective or that my feature works
• n/a 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:

• Mail helper initialization parameters are now optional (all parameters are still required for a successful API call, but they are not silently discarded if one is specified but others are not)

This is a stop-gap measure prior to the Mail helper being rewritten for a future update; the API ultimately returns errors if an element isn't specified regardless, so this makes the initialization a little more flexible (allows specifying a to_email at initialization to take advantage of automatic Personalization creation while still specifying the content later, for instance).

[SendGridDX]

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[thinkingserious]

Hello @onecrayon,

Thanks again for the PR!

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