uucore/parse_time: return 1ns for small numbers

[cakebaker]

This PR early returns 1ns for small numbers.

Fixes #8812

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8821 will not alter performance

_{Comparing cakebaker:uucore_parse_time_very_small_number (c05aca8) with main (788bf92)}

Summary

✅ 79 untouched
⏩ 73 skipped¹

Footnotes

1. 73 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩