fix: stdev calculation for less than 2 runs

[not-matthias]

No description provided.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #100 will degrade performances by 18.88%

_{Comparing cod-1051-api-validation-error-handling-stdev-is-null-for-divan (b6a7be8) with main (5caa771)}

Summary

❌ 4 regressions
✅ 159 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	bench_array1[4]	125.3 ns	154.4 ns	-18.88%
❌	bench_array2[4]	125.3 ns	154.4 ns	-18.88%
❌	init_array[4]	125.3 ns	154.4 ns	-18.88%
❌	mut_borrow	895.8 ns	925 ns	-3.15%

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #100 will degrade performances by 87.46%

_{Comparing cod-1051-api-validation-error-handling-stdev-is-null-for-divan (b6a7be8) with main (5caa771)}

Summary

⚡ 10 improvements
❌ 10 regressions
✅ 125 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	large_drop	169.7 µs	185.2 µs	-8.36%
⚡	large_setup	13 ns	12 ns	+8.33%
⚡	from_elem[2048]	256 ns	245 ns	+4.49%
⚡	from_elem[4096]	332 ns	317 ns	+4.73%
❌	generate_combinations[6]	1.3 µs	2.6 µs	-48.39%
❌	generate_combinations[8]	3.9 µs	4 µs	-3.88%
⚡	generate_parentheses[5]	21.9 µs	20.6 µs	+6.47%
⚡	graph_coloring[3]	2.4 µs	1 µs	×2.3
❌	n_queens_solver[5]	7.9 µs	8.5 µs	-7.62%
⚡	permutations[5]	13.4 µs	10.6 µs	+25.59%
⚡	rat_in_maze[6]	5.8 µs	2.9 µs	+98.41%
❌	add_two_integers[(0, 0)]	5.6 µs	5.9 µs	-5.66%
❌	generate_gray_code[1]	1 µs	4.3 µs	-75.7%
❌	generate_gray_code[2]	920 ns	3,802 ns	-75.8%
❌	generate_gray_code[3]	455 ns	3,629 ns	-87.46%
❌	generate_gray_code[4]	911 ns	3,632 ns	-74.92%
⚡	instant	5.1 µs	4.9 µs	+3.4%
❌	sleep_100ns	55.1 µs	57.6 µs	-4.35%
⚡	sleep_100us	129.4 µs	115.5 µs	+12.06%
⚡	sleep_1ns	57.7 µs	38.1 µs	+51.35%