Enable more clippy lints (part 1/3)

[emilk]

Which issue does this PR close?

• Part of Stricter Clippy checks in CI #18467.

Rationale for this change

Continuing the work in:

• Enable more clippy lints #24466
• Enable more clippy lints #24566
• Enable more clippy lints #24837

This PR was too big, so it is now part 1 of 3. The other parts are stacked on top of it:

• Enable more clippy lints (part 2/3) #25080
• Enable more clippy lints (part 3/3) #25081

Turn on more clippy::pedantic lints from the opt-out list in Cargo.toml.

What changes are included in this PR?

One commit per lint, each removing its "allow" line from Cargo.toml and
fixing every site. Review one commit at a time!

Let me know if you disagree with any and I'll revert it.

Lint	Sites	Fix
manual_string_new	84	String::new() instead of "".to_string() / "".to_owned() / "".into()
ignored_unit_patterns	52	Ok(()) instead of Ok(_), so the pattern stops matching if a payload is ever added
redundant_else	48	dropped else after a diverging branch

What is the testing strategy for this PR?

Clippy is clean both with --all-features and with default features (the latter
is what caught a cfg-gated site). The changes are mechanical, so no new tests.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.42444% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.74%. Comparing base (a5c809f) to head (33b2e9e).

Files with missing lines	Patch %	Lines
datafusion/expr/src/type_coercion/functions.rs	54.54%	5 Missing ⚠️
datafusion/sql/src/select.rs	85.71%	2 Missing and 3 partials ⚠️
datafusion/datasource-parquet/src/sink.rs	86.66%	2 Missing and 2 partials ⚠️
datafusion/execution/src/async_stream.rs	40.00%	1 Missing and 2 partials ⚠️
datafusion/spark/src/function/string/char.rs	0.00%	3 Missing ⚠️
benchmarks/src/cancellation.rs	0.00%	2 Missing ⚠️
datafusion/common/src/scalar/mod.rs	71.42%	2 Missing ⚠️
datafusion/functions-aggregate/src/first_last.rs	0.00%	2 Missing ⚠️
datafusion/common/src/error.rs	75.00%	1 Missing ⚠️
datafusion/datasource/src/memory.rs	0.00%	1 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24848      +/-   ##
==========================================
- Coverage   81.74%   81.74%   -0.01%     
==========================================
  Files        1128     1128              
  Lines      416644   416646       +2     
  Branches   416644   416646       +2     
==========================================
- Hits       340592   340589       -3     
- Misses      55995    55998       +3     
- Partials    20057    20059       +2     

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[emilk]

…this might be a bit big 😬 let me know if I should break it up.

(each commit in itself is pretty reviewable though)

[emilk]

Split into three parts, one PR each:

• Enable more clippy lints (part 1/3) #24848 (this one)
• Enable more clippy lints (part 2/3) #25080
• Enable more clippy lints (part 3/3) #25081