Fix NPE bug in block.toString() function

[frankfliu]

Change-Id: I2aa8bc6c8d36d01cc00c02778907b040d782330c

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 #1076 (122c355) into master (7a7f41f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1076      +/-   ##
============================================
- Coverage     70.05%   70.03%   -0.02%     
+ Complexity     5231     5229       -2     
============================================
  Files           511      511              
  Lines         23339    23341       +2     
  Branches       2492     2493       +1     
============================================
- Hits          16350    16347       -3     
- Misses         5660     5663       +3     
- Partials       1329     1331       +2     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/nn/AbstractBlock.java	64.51% <100.00%> (+0.46%)	⬆️
api/src/main/java/ai/djl/repository/Artifact.java	89.65% <0.00%> (-1.73%)	⬇️
.../main/java/ai/djl/benchmark/AbstractBenchmark.java	91.22% <0.00%> (-1.17%)	⬇️
...mark/src/main/java/ai/djl/benchmark/Arguments.java	78.08% <0.00%> (-0.57%)	⬇️
...n/java/ai/djl/tensorflow/engine/TfSymbolBlock.java	88.65% <0.00%> (ø)

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