fix(cloud_firestore): allow negative timestamp values

[dackers86]

Description

Negative values would throw an error with a value less than 0. This now allows for positive and negative values within a valid range.

Related Issues

Fixes #3545

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[mikehardy]

LGTM - the related issue was quite informative

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[mikehardy]

I don't the CLA bot is about me, I have one on file and have successfully submitted PRs here 🤔

[Salakar]

@dackers86 can you double check you've signed the CLA again please, it might be due to a changed GitHub email or something.

[kroikie]

@mikehardy could you try signing the CLA once more?

[mikehardy]

@googlebot I fixed it.

(apparently github@mikehardy.net was not one of my alternate emails in my google organization for this user, but (weirdly!) I have successfully merged PRs here before. Go figure :-) )

[mikehardy]

@kroikie CLA all set now, I'm confused how it happened but I was able to fix. Onwards!