Check the value of a global account manager before it is transformed

[cgsunkel]

Description of change

This PR fixes a bug that causes Data Hub to hang and eventually leads to a 504 error when a user clicks on ‘View full business details’ on affected one list companies (this also appears in Sentry). An example of this can be found here - an error message should appear in the 'data-hub-sentry' channel after the link is clicked.

The bug is caused if someone using Django Admin changes a company to the one list tier but does not set the one list account owner. In the code, the translator always expects one_list_global_account_manager to be defined and throws an unexpected error when encountering 'null'

This PR allows the transformer to check the value of one_list_global_account_manager before the transformation process starts, and returns a value of ‘not set’ if the variable hasn’t been set beforehand.

Test instructions

Go the the company identified above in the Heroku app and the business details page should appear normally

Screenshots

Before

After

A normal business details page should appear

Checklist

• Has the branch been rebased to master?
• Automated tests (Any of the following when applicable: Unit, Functional or Acceptance)
• Manual compatibility testing (Browsers: Chrome, Firefox, IE11, Safari)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@bf18b32). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2474   +/-   ##
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  Coverage          ?   87.41%           
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  Files             ?      336           
  Lines             ?     5922           
  Branches          ?        0           
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  Hits              ?     5177           
  Misses            ?      745           
  Partials          ?        0

Impacted Files	Coverage Δ
...ps/companies/apps/business-details/transformers.js	100% <100%> (ø)

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