fix: Accept empty html on Email helper NewSingleEmail()

[tjun]

Fixes #266

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:

• Accept Empty HTML content on Email helper NewSingleEmail function

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

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The diff coverage is n/a.

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##             main     #272   +/-   ##
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  Coverage        ?   71.14%           
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  Files           ?        3           
  Lines           ?      454           
  Branches        ?        0           
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  Hits            ?      323           
  Misses          ?      127           
  Partials        ?        4           

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