MDEV-20022 sql_mode="oracle" TO_NUMBER - fix clang compile

[grooverdan]

• The Jira issue number for this PR is: MDEV-20022

Description

Clang(21.5) issues the following warning and suggestion that has been applied:

sql/item_numconvfunc.cc:2447:15: error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]
2447 | std::move(Zeros_or_nines(static_cast<Zeros&&>(rhs))),
| ^
sql/item_numconvfunc.cc:2447:15: note: remove std::move call here
2447 | std::move(Zeros_or_nines(static_cast<Zeros&&>(rhs))),
| ^~~~~~~~~~

Release Notes

nothing on this change.

How can this PR be tested?

amd64-msan-clang-20-debug and amd64-ubasan-clang-20-debug have been failing since b10e209 was pushed.

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

func_numconv still passes with this change.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.