feat: allow argument-less TemplateExpression parts

[MilesCranmer]

No description provided.

[github-actions]

Benchmark Results

	master	f4c0d7c...	master/f4c0d7c51b83fa...
search/multithreading	23.2 ± 1.1 s	21.3 ± 1.7 s	1.09
search/serial	35 ± 0.33 s	35.2 ± 0.73 s	0.993
utils/best_of_sample	1.76 ± 1.6 μs	1.69 ± 1.5 μs	1.04
utils/check_constraints_x10	12.2 ± 3.2 μs	12.1 ± 3.2 μs	1.01
utils/compute_complexity_x10/Float64	2.09 ± 0.11 μs	2.08 ± 0.11 μs	1
utils/compute_complexity_x10/Int64	2.06 ± 0.11 μs	2.08 ± 0.11 μs	0.99
utils/compute_complexity_x10/nothing	1.42 ± 0.1 μs	1.43 ± 0.13 μs	0.994
utils/insert_random_op_x10	5.86 ± 1.9 μs	5.73 ± 1.8 μs	1.02
utils/next_generation_x100	0.44 ± 0.12 ms	0.352 ± 0.12 ms	1.25
utils/optimize_constants_x10	0.0389 ± 0.0094 s	0.0384 ± 0.0086 s	1.01
utils/randomly_rotate_tree_x10	5.22 ± 0.58 μs	5.28 ± 0.6 μs	0.989
time_to_load	1.83 ± 0.039 s	1.83 ± 0.0067 s	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[coveralls]

Pull Request Test Coverage Report for Build 12059612888

Details

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 95.467%

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Totals
Change from base Build 11993147481:	0.005%
Covered Lines:	3117
Relevant Lines:	3265

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💛 - Coveralls