GH-51673: [C++][Parquet] Skip missing min/max in metadata printer

[Sunny-Island]

Rationale for this change

Parquet column statistics can contain a null count without min/max values, for example when every value in a column chunk is null. ParquetFilePrinter::DebugPrint currently formats min and max whenever any statistics are present. For a DECIMAL stored as FIXED_LEN_BYTE_ARRAY, formatting the missing max passes an empty byte array to Decimal256::FromBigEndian and makes parquet-reader --only-metadata fail.

Closes #51673.

What changes are included in this PR?

• Print min and max only when their respective encoded statistics are present. Keep the existing output for columns with both values.
• Add a self-contained reader test that writes an all-null DECIMAL column, verifies that null-count statistics exist without min/max, and checks that the text printer omits both values.

Are these changes tested?

• parquet-reader-test --gtest_filter=TestDebugPrintWithMemoryFile.AllNullDecimalStatistics:TestAllTypesPlain.DebugPrintWorks
• parquet-reader --only-metadata against the Open Prices prices.parquet file referenced in the issue: all 313 row groups print successfully.

Are there any user-facing changes?

parquet-reader --only-metadata now prints valid files with all-null DECIMAL column chunks without an error. Missing min/max values are omitted instead of being formatted.

Was AI used for this PR?

In accordance with the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

AI assisted with the code, test, and PR description. The issue was reproduced on the public Parquet file and investigated at the throw site with LLDB.

• GitHub Issue: [C++][Parquet] Error when printing file metadata #51673