Shifted instances: make sure ref_instances is not empty

[getzze]

Description

Correction of another bug introduced by save_shifted_instances=True. Hopefully the last one...
I had corrected it locally but forgot to include it in the last correction PR #1001.

Loading frames from self.shifted_instances, ref_instances changes and can become an empty list.
But the test for ref_instances being non-empty was before these lines, therefore the following lines crash because ref_instances is unexpectedly an empty list.
There are two changes to correct this bug:

• put the code for loading from self.shifted_instances above the len(ref_instances) > 0 test (this is not really needed but it makes the code easier to read).
• ref_instances (and ref_img) is modified only if it is not empty.

Types of changes

• Bugfix

[codecov]

Codecov Report

Merging #1029 (bc25a5f) into develop (f54e6fc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1029   +/-   ##
========================================
  Coverage    66.74%   66.74%           
========================================
  Files          127      127           
  Lines        21526    21527    +1     
========================================
+ Hits         14367    14368    +1     
  Misses        7159     7159           

Impacted Files	Coverage Δ
sleap/nn/tracking.py	72.81% <100.00%> (+0.05%)	⬆️

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[roomrys]

lgtm