Leave non-dated issues for last when sorting by by nearest due date

[guillep2k]

"Sort by nearest due date" should leave the ones without due date for last. For example:

Currently:

#_2324          (no due date)
#_2626          (no due date)
#_1234    Oct 3 (past due)
#_1432    Nov 7 (due Today)
#_5232    Nov 8 (due Tomorrow)

Expected:

#_1234    Oct 3 (past due)
#_1432    Nov 7 (due Today)
#_5232    Nov 8 (due Tomorrow)
#_2324          (no due date)
#_2626          (no due date)

This PR fixes that.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1f3ba69). Click here to learn what that means.
The diff coverage is 0%.

@@           Coverage Diff            @@
##             master   #8871   +/-   ##
========================================
  Coverage          ?   41.2%           
========================================
  Files             ?     544           
  Lines             ?   70096           
  Branches          ?       0           
========================================
  Hits              ?   28886           
  Misses            ?   37514           
  Partials          ?    3696

Impacted Files	Coverage Δ
models/issue.go	51.2% <0%> (ø)

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