fix for test failure in main

[shanbady]

What are the relevant tickets?

fixes test failure on main https://github.com/mitodl/mit-open/actions/runs/8332224748/job/22800891647?pr=630

Description (What does it do?)

sets external models setting to empty so test can create programcertficate objects

How can this be tested?

Tests should succeed

Additional Context

PR #617 locks down programcertficate tables as readonly. this PR was merged in after another PR that had tests for programcertificates

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.83%. Comparing base (1b9c0ce) to head (7e598bf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #631       +/-   ##
===========================================
+ Coverage   50.34%   74.83%   +24.48%     
===========================================
  Files         102      246      +144     
  Lines        4626    11214     +6588     
  Branches      979     1948      +969     
===========================================
+ Hits         2329     8392     +6063     
- Misses       2242     2648      +406     
- Partials       55      174      +119     

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