[C++] Allocate memory of a random size because the passed value is an uninitialized int32_t.

[flashzxi]

Describe the bug, including details regarding any error messages, version, and platform.

In grouper.cc, allocated a random uint32_t size memory because of using uninitialized offset vector when group_nums == 0

see pr: https://github.com/apache/arrow/pull/43123

Component(s)

C++

[flashzxi]

lines 780-782 were added by me

[zanmato1984]

Issue resolved by pull request 43123 https://github.com/apache/arrow/pull/43123