[api] Fix saveModel NPE bug

[frankfliu]

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 #989 (60b6289) into master (62c0770) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master     #989   +/-   ##
=========================================
  Coverage     70.02%   70.02%           
- Complexity     5085     5086    +1     
=========================================
  Files           502      502           
  Lines         22519    22521    +2     
  Branches       2359     2360    +1     
=========================================
+ Hits          15768    15770    +2     
  Misses         5495     5495           
  Partials       1256     1256           

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/BaseModel.java	73.64% <0.00%> (-1.01%)	⬇️
...c/main/java/ai/djl/mxnet/engine/MxSymbolBlock.java	83.45% <0.00%> (+1.43%)	⬆️

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