fix(types): return nil duration and timestamp

[ryanchristo]

Description

Closes: #1272

Manual tests: https://hackmd.io/ioZPdD22REaHBE1YQg1YPg

This pull request updates timestamp conversion functions to return nil if timestamp or duration is nil rather than returning the zero-value timestamp and duration. This only affects query responses where a timestamp or duration is used and where these values are not required when being added to state (e.g. sell order expiration). This will also use nil rather than the zero-value timestamp for issuance date when querying batches if a batch is added between now and the upgrade.

For example, a sell order with no expiration previously returned:

sell_order:
  ask_amount: "1000000"
  ask_denom: stake
  batch_denom: C01-001-20200101-20210101-001
  disable_auto_retire: true
  expiration: "1970-01-01T00:00:00Z"
  id: "2"
  quantity: "100"
  seller: regen1mjj52fal48xsj2q3yypguzwuqccxfr3larwezy

Now, a sell order with no expiration returns:

sell_order:
  ask_amount: "1000000"
  ask_denom: stake
  batch_denom: C01-001-20200101-20210101-001
  disable_auto_retire: true
  expiration: null
  id: "2"
  quantity: "100"
  seller: regen1mjj52fal48xsj2q3yypguzwuqccxfr3larwezy

---

Author Checklist

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• confirmed the correct type prefix in the PR title
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[codecov]

Codecov Report

Merging #1292 (f6ef2d2) into master (5ce8e9e) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1292      +/-   ##
==========================================
+ Coverage   77.51%   77.58%   +0.07%     
==========================================
  Files         213      213              
  Lines       16084    16084              
==========================================
+ Hits        12467    12479      +12     
+ Misses       2613     2597      -16     
- Partials     1004     1008       +4     

Flag	Coverage Δ
experimental-codecov	77.61% <100.00%> (+0.04%)	⬆️
stable-codecov	77.35% <100.00%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
types/timestamp.go	40.74% <100.00%> (ø)
types/ormstore/store.go	39.73% <0.00%> (+3.90%)	⬆️

[technicallyty]

lgtm!