Fix `userSignatureForNow` sometimes returning nil.

On the iOS simulator, `NSFullUserName()` (and presumably
`NSUserName()`) return an empty string; currently we check for nil.
This leads to `userSignatureForNow` returning nil and, eventually, a
crash. Instead make sure that `NSFullUserName()` and `NSUserName()`
don’t return an empty string or nil.
Use a simpler check.
It's == not !=.
Repeated calls are wasteful.
Merge pull request #328 from PiersonBro/fix-userSignatureForNow

Fix `userSignatureForNow` sometimes returning nil.