Don't error on empty typeinf flamegraph; print a warning instead

[NHDaly]

See #212 (comment).

[codecov]

Codecov Report

Base: 83.56% // Head: 83.56% // No change to project coverage 👍

Coverage data is based on head (0ad1e5f) compared to base (4e00e22).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   83.56%   83.56%           
=======================================
  Files          17       17           
  Lines        2178     2178           
=======================================
  Hits         1820     1820           
  Misses        358      358           

Impacted Files	Coverage Δ
src/parcel_snoopi_deep.jl	90.01% <100.00%> (ø)

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