[bugfix] wrap read metadata.json in a try except block

[NihalHarish]

Description of changes:

• if the metadata.json file is empty there is potential that the hook will crash.

Style and formatting:

I have run pre-commit install to ensure that auto-formatting happens with every commit.

Issue number, if available

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[codecov-io]

Codecov Report

Merging #466 (b3e6068) into master (433348d) will decrease coverage by 0.41%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   65.62%   65.21%   -0.42%     
==========================================
  Files         172      162      -10     
  Lines       13260    12918     -342     
==========================================
- Hits         8702     8424     -278     
+ Misses       4558     4494      -64     

Impacted Files	Coverage Δ
smdebug/core/state_store.py	79.16% <40.00%> (-1.87%)	:arrow_down:
smdebug/core/modes.py	55.00% <0.00%> (-20.00%)	:arrow_down:
smdebug/xgboost/singleton_utils.py	0.00% <0.00%> (-20.00%)	:arrow_down:
smdebug/trials/profiler_trial.py	24.61% <0.00%> (-18.47%)	:arrow_down:
smdebug/mxnet/collection.py	73.33% <0.00%> (-16.67%)	:arrow_down:
smdebug/exceptions.py	65.47% <0.00%> (-15.48%)	:arrow_down:
smdebug/core/reader.py	85.18% <0.00%> (-7.41%)	:arrow_down:
smdebug/tensorflow/callable_cache.py	78.26% <0.00%> (-6.53%)	:arrow_down:
smdebug/core/locations.py	86.11% <0.00%> (-5.56%)	:arrow_down:
smdebug/core/tensor.py	79.03% <0.00%> (-2.02%)	:arrow_down:
... and 12 more

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