Fix na casting behavior for groupby.agg with arrow dtypes

[phofl]

• Closes When using PyArrow dtypes, aggregations create NaNs of unexpected type #11116
• Tests added / passed
• Passes pre-commit run --all-files

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

     15 files  ± 0       15 suites  ±0   3h 22m 17s ⏱️ - 5m 11s
 13 125 tests + 2   12 191 ✅ ±0     931 💤 ±0  3 ❌ +2 
162 516 runs  +24  142 463 ✅ +8  20 044 💤 +8  9 ❌ +8 

For more details on these failures, see this check.

Results for commit 3ae8d40. ± Comparison against base commit f511e93.