fix latexify on complex expressions

[mxhbl]

At the moment, latexify fails on some complex expressions involving Symbolics.
For example,

using Symbolics, Latexify
latexify(im * Symbolics.Term(sqrt, [2]))

currently results in an error.

This PR is a small fix that resolves this error with minimal changes to the existing code. More work is probably needed to ensure complex pre-factors always display nicely.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.90%. Comparing base (47cfd60) to head (9cba874).
Report is 21 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1238       +/-   ##
===========================================
+ Coverage    8.30%   79.90%   +71.59%     
===========================================
  Files          46       46               
  Lines        4549     4633       +84     
===========================================
+ Hits          378     3702     +3324     
+ Misses       4171      931     -3240     

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[ChrisRackauckas]

It's at least a reasonable start, thanks!