Fix nullable model properties in from_dict and to_dict

[forest-benchling]

Currently, the code only accounts for the not required case, not the nullable case.

This PR also adds E2E test coverage.

[forest-benchling]

cc @dbanty. Sorry, I know you're busy :) We're planning to release our SDK's first version this Friday, so we'll just merge this into our fork if you don't have time to get to it by then.

[codecov]

Codecov Report

Merging #315 (3a11f0d) into main (398692d) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## main #315 +/- ##
=========================================
Coverage 100.00% 100.00% =========================================
Files 47 47 Lines 1385 1385 =========================================
Hits 1385 1385 

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[forest-benchling]

(cc @bowenwr)

[dbanty]

You've been busy! 😅 This looks good to me, thanks! But I've reviewed several things in a row so I'm not completely trusting my brain 🥱. Gonna request that @emann gives this a second look since it's already past Friday and therefore probably not a rush.

Thanks again, hopefully I'll get around to all your other PRs soon!

[emann]

Looks good to me as well - thanks for all of your contributions @forest-benchling!