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MINOR: [C++][Parquet] fix dict_length for ReadDictionary when not having dict (#41344)



### Rationale for this change

`dictionary_length = 0` is used when page doesn't have dictionary, however, this should be a nop.

### What changes are included in this PR?

Change `dictionary_length = 0` to `*dictionary_length = 0`.

### Are these changes tested?

No?

### Are there any user-facing changes?

no

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>