ARROW-17973: [C++] Expression::ToString wrong for nullary function call

[sanjibansg]

This PR fixes the ToString() method in the Expression class to handle functions with no arguments.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-17973

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[pitrou]

Thanks a lot @sanjibansg . Just one suggestion below.

[ursabot]

Benchmark runs are scheduled for baseline = 3d7f2f2 and contender = 7be5e95. 7be5e95 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.56% ⬆️0.0%] test-mac-arm
[Failed ⬇️0.27% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.07% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 7be5e95e ec2-t3-xlarge-us-east-2
[Failed] 7be5e95e test-mac-arm
[Failed] 7be5e95e ursa-i9-9960x
[Finished] 7be5e95e ursa-thinkcentre-m75q
[Finished] 3d7f2f22 ec2-t3-xlarge-us-east-2
[Failed] 3d7f2f22 test-mac-arm
[Failed] 3d7f2f22 ursa-i9-9960x
[Finished] 3d7f2f22 ursa-thinkcentre-m75q
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