fix: don't issue atomic id completions when there is a dangling dot

[mhuisi]

This PR fixes an old auto-completion bug where x. would issue nonsensical completions when x. could not be elaborated as a dot completion.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f0c190239a5e2d7f99413da0c54a575a0d0353b6 --onto 193b6f2bec332ac0bce33e10856a96163d4be456. (2024-10-25 13:10:37)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4bef3588b592c1cdcba190ef9bcd92b05fd5cbb7 --onto 5a99cb326c9e9052e135cc63ed02b63371cff6d3. (2024-11-19 09:02:23)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d6f898001bbb34604797a495d18803a9df584106 --onto 4600bb16fcded0356d20ae232e7f8580c56a5955. (2024-11-19 12:21:25)