fix: Handle nulls correctly when extracting nested arrays from nullable struct/map

[AdamGS]

Which issue does this PR close?

• Closes Nested field access on nullable structs discard parent nulls #25120.

Rationale for this change

Fixes null handling when accessing nested map/struct arrays by AND-ing the null buffers as we go down into the array tree.

Running the get_field benchmarks on my M3 macbook, one micro benchmark has a small repeatable regression:

get_field_map_1024_entries_4_first
                        time:   [18.811 µs 18.881 µs 18.954 µs]
                        change: [+6.4039% +6.8632% +7.3094%] (p = 0.00 < 0.05)
                        Performance has regressed.

What changes are included in this PR?

Taking parent nulls into account when extracting nested arrays from map/struct arrays.

What is the testing strategy for this PR?

Additional unit tests and SLT tests.

Are there any user-facing changes?

Correct behavior.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.80%. Comparing base (4048898) to head (8a137f8).

Files with missing lines	Patch %	Lines
datafusion/functions/src/core/getfield.rs	84.94%	3 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25122      +/-   ##
==========================================
- Coverage   81.80%   81.80%   -0.01%     
==========================================
  Files        1130     1130              
  Lines      417754   417843      +89     
  Branches   417754   417843      +89     
==========================================
+ Hits       341754   341820      +66     
- Misses      55875    55881       +6     
- Partials    20125    20142      +17     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.