[api] Avoid NPE in Metric.toString()

[frankfliu]

Change-Id: I5dae8088aff74f5d77558ab30aa5e8d0fb96926b

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 #1626 (425b683) into master (bb5073f) will decrease coverage by 1.20%.
The diff coverage is 62.11%.

@@             Coverage Diff              @@
##             master    #1626      +/-   ##
============================================
- Coverage     72.08%   70.87%   -1.21%     
- Complexity     5126     5430     +304     
============================================
  Files           473      507      +34     
  Lines         21970    23760    +1790     
  Branches       2351     2589     +238     
============================================
+ Hits          15838    16841    +1003     
- Misses         4925     5629     +704     
- Partials       1207     1290      +83     

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%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...ain/java/ai/djl/ndarray/index/dim/NDIndexPick.java	100.00% <ø> (ø)
api/src/main/java/ai/djl/nn/Blocks.java	75.00% <0.00%> (-25.00%)	⬇️
... and 238 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 05a685e...425b683. Read the comment docs.