Fix bug when passing metrics as empty list

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #660 (559a5e4) into master (9061d4b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #660   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files         173      173           
  Lines        9232     9232           
=======================================
  Hits         8363     8363           
  Misses        869      869           

Flag	Coverage Δ
gpu	?
pytest	?
unittests	90.58% <100.00%> (ø)

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Impacted Files	Coverage Δ
flash/core/classification.py	96.52% <100.00%> (ø)
flash/image/classification/model.py	76.92% <100.00%> (ø)
flash/text/classification/model.py	93.18% <100.00%> (ø)

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[Borda]

not sure if I see the fix? mind highlight it...

[tchaton]

LGMT !