fix(completion): correct behavior of slice past end of array

[reubeno]

When using the substring/slice operator past the end of an array, make sure not to yield an empty string.

[github-actions]

Test Results

  2 files    9 suites   1m 5s ⏱️
498 tests 498 ✅ 0 💤 0 ❌
985 runs  985 ✅ 0 💤 0 ❌

Results for commit 6c17c42.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.42 μs	3.42 μs	0.00 μs	⚪ Unchanged
instantiate_shell	60.71 μs	60.08 μs	-0.63 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	31094.64 μs	31875.78 μs	781.14 μs	⚪ Unchanged
parse_bash_completion	2783.53 μs	2733.24 μs	-50.29 μs	🟢 -1.81%
parse_sample_script	4.17 μs	4.17 μs	0.00 μs	⚪ Unchanged
run_echo_builtin_command	89.89 μs	89.98 μs	0.10 μs	⚪ Unchanged
run_one_builtin_command	107.47 μs	108.80 μs	1.33 μs	⚪ Unchanged
run_one_external_command	1955.11 μs	1985.91 μs	30.80 μs	⚪ Unchanged
run_one_external_command_directly	1018.09 μs	1014.64 μs	-3.45 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/expansion.rs	🟢 95.92%	🟢 95.91%	🔴 -0.01%
brush-core/src/shell.rs	🟢 78.66%	🟢 78.52%	🔴 -0.14%
Overall Coverage	🟢 73.56%	🟢 73.55%	🔴 -0.01%

Minimum allowed coverage is 70%, this run produced 73.55%