fix: use unit for fmtstr without variables

[asterite]

Description

Problem

Resolves #9729

Summary

Alternative to #10003

When I found this issue I also thought one way to fix this was to have Unit be the same as an empty Tuple, meaning removing the Unit type.

However, another solution could be to special case a fmtstr with no variables to have Unit be the type instead of an empty tuple. In fact this is what we do with a function's Env, which is Unit when it doesn't capture any variables:

noir/compiler/noirc_frontend/src/elaborator/expressions.rs

Lines 1390 to 1391 in e422898

	let env_type =
	if captured_vars.is_empty() { Type::Unit } else { Type::Tuple(captured_vars) };

Then we need to handle Unit in a couple of places, but that's it.

Additional Context

User Documentation

Check one:

• No user documentation needed.
• Changes in docs/ included in this PR.
• [For Experimental Features] Changes in docs/ to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: a219e7a	Previous: e422898	Ratio
test_report_zkpassport_noir-ecdsa_	2 s	1 s	2

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench