MINOR: [C++][Parquet] fix dict_length for ReadDictionary when not hav…

…ing dict (apache#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>

cpp/src/parquet/column_reader.cc

@@ -1340,7 +1340,7 @@ class TypedRecordReader : public TypedColumnReaderImpl<DType>,
 
  const void* ReadDictionary(int32_t* dictionary_length) override {
  if (this->current_decoder_ == nullptr && !this->HasNextInternal()) {
- dictionary_length = 0;
+ *dictionary_length = 0;
  return nullptr;
  }
  // Verify the current data page is dictionary encoded. The current_encoding_ should