#638 Add default value for SubstitutionTag parameter

[KoditkarVedant]

Fixes

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:

• The SendGridMessage has the SetOpenTracking which takes two arguments enable and substitutionTag. The substitutionTag argument is optional and if user don't want to use it currently, they need to pass null explicitly as we have it as required. So I've set it to have default null value.

[SendGridDX]

All committers have signed the CLA.

[KoditkarVedant]

Hello @thinkingserious, can you please look into this PR?

[thinkingserious]

This PR LGTM. It's now on our backlog for merging. Thanks!

[thinkingserious]

Hello @KoditkarVedant,

Thanks again for the PR!

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