Fix enum loading bug

[FraserGreenroyd]

Issues addressed by this PR

Fixes #363

Test files

N/A - test steps instead:

1. Create an Excel file with a BHoM enum object.
2. Save the file
3. Change the enum which was added to the Excel sheet by adding or removing a value
4. Recompile the enum
5. Reopen the Excel file previously made
6. Verify the dropdown is now updated with the changed values - this did not happen on main

Changelog

Additional comments

[FraserGreenroyd]

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[FraserGreenroyd]

@BHoMBot check core @BHoMBot check null-handling @BHoMBot check serialisation

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