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GH-43124: [C++] Initialize offset vector head as 0 after memory allocated in grouper.cc (#43123)

## bug description
In grouper.cc:780, memory is allocated for offset vector of varlen column, however the vector is initialized in  `encoder_.DecodeFixedLengthBuffers`, which will never be called when `num_groups==0`(see line 786). Then `fixedlen_bufs[i][0]` will be a uninitialized value that means a random uint32_t. Later this random uint32_t is used to `AllocatePaddedBuffer(varlen_size)`. However an random uint32_t is up to 4GB memory, the program may run normally without being affected.

## how to fix
set offset vector head as 0 after memory allocated in case it won't be initialized when `num_groups==0`

* GitHub Issue: #43124

Lead-authored-by: FlashZXi <flashzxi@outlook.com>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Co-authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Ruoxi Sun <zanmato1984@gmail.com>