Don't compare two blank moment() for potential race condition

[harumhl]

#10225

Changes

When appearanceSets[setKey].availableUntil is undefined, then we were performing moment().isBefore(moment()) and the race condition occasionally returns true about 0.05-0.5% of the time (from my experience).

How to test:

1. Go to website/client/src/mixins/avatarEditUtilities.js
2. Change hide = this.hideSet(set); to hide = this.hideSet(set, key);
3. Change hideSet (setKey) to hideSet (setKey, key)
4. Add below code after if (appearanceSets[setKey].availableFrom) block

      if (key === 'bear') {
        for (let i = 0; i < 10000; i += 1) {
          const tempHide = moment(appearanceSets[setKey].availableUntil).isBefore(moment());
          if (tempHide) {
            console.log(tempHide);
          }
        }
      }

5. Run the local instance of Habitica
6. Click User icon > Click 'Edit Avatar' > Click 'Skin'
7. You should see a few true cases on Console
8. Now change the above const tempHide = ... with below block (which has the identical logic as this PR)

          const tempHide = appearanceSets[setKey].availableUntil
            ? moment(appearanceSets[setKey].availableUntil).isBefore(moment())
            : false;

9. Repeat steps 6
10. Now you don't see any true printed out on Console

• The key above doesn't have to be 'bear'. I just wanted a key that will return hide to be false by default. You can try any key for Skin that should return hide = false (e.g. 'rainbow')

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