Optimize returned plan in roundtrip_fill_na function

[nseekhao]

Which issue does this PR close?

Closes #4897

Rationale for this change

The test that failed used a function that compared an optimized plan to an unoptimized plan.

While at it, also changed from using the deprecated SessionContext::optimize to the recommended SessionState::optimize.

What changes are included in this PR?

Modified the roundtrip_fill_na() function used by the failed test to compare optimized plan to optimized plan. Test now passes.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[ursabot]

Benchmark runs are scheduled for baseline = 7e94826 and contender = 3a1f033. 3a1f033 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java