Change timestamp fieldType to be nullable

[nekketsuuu]

#162

Since datumParserTimestamp can return nil value, fieldType also should allow nil.
Without this, nil timestamp will be recognized as a strange timestamp, such as 1754-08-30 UTC (-6795364578871).

Follow-up: 4750ff8

Screenshots

Before
After

The second record contains a null value in clock measure, which is displayed as 1754-08-31 in the first screenshot and as an empty cell in the second screenshot.

How to reproduce these screenshots

1. Insert above data points to a newly created table. I used this Ruby script: https://gist.github.com/nekketsuuu/c84479128ca9487be7c018c741732be1
2. Create a Table panel on Grafana, with a query select * from $__database.$__table where $__timeFilter.

Tests

Currently I marked this pull request as draft because I couldn't update test files. It's difficult to me to update them for a reason described by #183. I'd appreciate if someone would tell me how to regenerate golden files from scratch.

Solved by 9e24fb7

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[sunker]

@nekketsuuu I have now re-generated the test files for you. Sorry it's been taking such a long time. Let us know when this PR is ready for review and we'll take a look. Thanks!

[nekketsuuu]

@sunker Awesome! Thank you for looking my pull request! I checked your changes and marked this pull request is ready for review.

[sunker]

Looks good! Thanks for contributing @nekketsuuu!