[api] Fix NPE bug in getContextClassLoader().

[frankfliu]

Change-Id: Icae3ae41116bcb79c8d665794cab787848f9583e

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 #1445 (e1a0241) into master (bb5073f) will decrease coverage by 1.42%.
The diff coverage is 47.98%.

@@             Coverage Diff              @@
##             master    #1445      +/-   ##
============================================
- Coverage     72.08%   70.66%   -1.43%     
- Complexity     5126     5297     +171     
============================================
  Files           473      493      +20     
  Lines         21970    23221    +1251     
  Branches       2351     2533     +182     
============================================
+ Hits          15838    16410     +572     
- Misses         4925     5531     +606     
- Partials       1207     1280      +73     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <ø> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../modality/cv/translator/YoloTranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...i/djl/modality/cv/translator/YoloV5Translator.java	5.69% <0.00%> (ø)
...odality/cv/translator/YoloV5TranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...modality/nlp/embedding/TrainableWordEmbedding.java	45.00% <0.00%> (-3.34%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...i/src/main/java/ai/djl/ndarray/NDArrayAdapter.java	15.23% <0.00%> (-0.83%)	⬇️
... and 159 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c41b6a0...e1a0241. Read the comment docs.