fix: handling keys with null values in @speckle/objectsender

[ocni-dtu]

Description & motivation

Bug fix to the @speckle/objectsender package, when an object key has null value.

I originally described the bug on the forum

The #traverse method was not handling null values are where throwing a TypeError value is null when reaching line 107 because .speckle_type couldn't be accessed.

I've changed it so that null values now are caught in line 60 together with other primitives.

I have also updated the test/example case to include a null value to verify my implementation.

Changes:

• Updated line 60.

Screenshots:

N/A

Validation of changes:

• Added nothing: null to generateTestObject()

Checklist:

• My pull request follows the guidelines in the Contributing guide?
• My pull request does not duplicate any other open Pull Requests for the same update/change?
• My commits are related to the pull request and do not amend unrelated code or documentation.
• My code follows a similar style to existing code.
• I have added appropriate tests.
• I have updated or added relevant documentation.