SNOW-1303970 Handle null values correctly in structured types

[sfc-gh-pfus]

Description

SNOW-1303970 Handling of null values in structured objects and maps

Checklist

• [x] Created tests which fail without the change (if possible)
• [x] Extended the README / documentation, if necessary

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 2.71739% with 358 lines in your changes are missing coverage. Please review.

Project coverage is 77.05%. Comparing base (4f7c965) to head (2e4450b). Report is 1 commits behind head on master.

Files	Patch %	Lines
structured_type.go	0.00%	197 Missing :warning:
converter.go	1.85%	159 Missing :warning:
util.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1093 +/- ## ========================================== - Coverage 79.24% 77.05% -2.20% ========================================== Files 52 52 Lines 10669 10949 +280 ========================================== - Hits 8455 8437 -18 - Misses 2214 2512 +298 ```

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