Avoid exception in NDArray.toString()

[frankfliu]

Change-Id: I75e32ba17bdab38d04318ae72cf18a45a1e1b5c3

Description

Brief description of what this PR is about

• If this change is a backward incompatible change, why must this change be made?
• Interesting edge cases to note here

[codecov-commenter]

Codecov Report

Merging #1157 (645a7f7) into master (002c04a) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #1157   +/-   ##
=========================================
  Coverage     70.20%   70.21%           
  Complexity     5342     5342           
=========================================
  Files           513      513           
  Lines         23703    23703           
  Branches       2562     2562           
=========================================
+ Hits          16641    16642    +1     
+ Misses         5697     5695    -2     
- Partials       1365     1366    +1     

Impacted Files	Coverage Δ
...i/src/main/java/ai/djl/ndarray/NDArrayAdapter.java	6.66% <0.00%> (ø)
api/src/main/java/ai/djl/repository/Artifact.java	86.17% <0.00%> (-1.63%)	⬇️
api/src/main/java/ai/djl/inference/Predictor.java	90.38% <0.00%> (+2.88%)	⬆️

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