Fix redundant jwt.decode() calls when JWT_GET_USER_SECRET_KEY is None

[fengsi]

In most cases, it does not make any sense to perform TWO jwt.decode(), just trying to get user_id even when it's not used. The logic is now updated so that it will be short-circuited.

Interestingly, I found a previous commit from the original contributor:
13ca172

It used jwt_get_secret_key(user_id=None) before. However, it did not avoid making two jwt.decode().

This PR fixed such issue. jwt.decode() is somewhat expensive call and we should not call it unnecessarily.

[codecov]

Codecov Report

Merging #416 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
- Coverage   90.67%   90.34%   -0.34%     
==========================================
  Files          14       12       -2     
  Lines         847      818      -29     
  Branches       29       29              
==========================================
- Hits          768      739      -29     
  Misses         66       66              
  Partials       13       13

Flag	Coverage Δ
#codecov	90.34% <ø> (-0.34%)	⬇️
#dj110	87.04% <ø> (-0.45%)	⬇️
#dj111	87.04% <ø> (-0.45%)	⬇️
#dj18	89.48% <ø> (-0.36%)	⬇️
#dj19	89.48% <ø> (-0.36%)	⬇️
#drf31	89.48% <ø> (-0.36%)	⬇️
#drf32	89.48% <ø> (-0.36%)	⬇️
#drf33	89.48% <ø> (-0.36%)	⬇️
#drf34	90.34% <ø> (-0.34%)	⬇️
#drf35	89.97% <ø> (-0.35%)	⬇️
#drf36	89.97% <ø> (-0.35%)	⬇️
#py27	90.34% <ø> (-0.34%)	⬇️
#py33	89.11% <ø> (-0.38%)	⬇️
#py34	89.97% <ø> (+0.48%)	⬆️
#py35	87.04% <ø> (?)
#py36	87.04% <ø> (?)

Impacted Files	Coverage Δ
rest_framework_jwt/models.py

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