Fixing scaling bug related to external functions with string arguments

[andrewlee94]

Related to #1355

Summary/Motivation:

The discussion in #1355 brought up an edge case where the nominal expression walker was failing due to encountering an external function with a string argument.

Changes proposed in this PR:

• Check for string arguments when evaluating external function nodes
• Adds test for this edge case

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.84%. Comparing base (6ffcde7) to head (edcfe2a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1487      +/-   ##
==========================================
- Coverage   76.84%   76.84%   -0.01%     
==========================================
  Files         376      376              
  Lines       61087    61091       +4     
  Branches    13504    13505       +1     
==========================================
+ Hits        46942    46945       +3     
- Misses      11763    11766       +3     
+ Partials     2382     2380       -2     

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[dallan-keylogic]

LGTM