[REF] export code simplification

[eileenmcnaughton]

Overview

Removes an unnecessary variable check, simplifies tests

Before

Less logical

After

More logical

Technical Details

On review I find the value ['exportOption'] is a parameter used at the form later to determine whether to
export 'Primary fields only' (ahem) or the user should select an array of fields. By the time the form is submitted we
either have an array of selected fields or NULL denoting that there is no selection (& hence the defaults should be used)
and the selection at the form layer is redundant. However it is still being referenced in one place (and hence the tests
are passing it in to avoid an enotice). This fixes that one place & removes it from the tests. It also extracts part of
a test to move towards making it easier to alter the signature of exportComponents without messing with so many places in
the code

Comments

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

test this please

[eileenmcnaughton]

@colemanw this & #14762 are small steps in the right direction

[colemanw]

I've reviewed the code and it seems to make sense.