Report generation crashes on empty categorical histograms

Priority Level

Medium (Annoying but has workaround)

Describe the bug

DatasetProfilerResults accepts categorical histogram data with empty categories and counts lists, but to_report() crashes while rendering it:

ValueError: max() arg is an empty sequence

This prevents report generation for a schema-valid analysis result with no categorical observations.

Steps/Code to reproduce bug

from pathlib import Path

from data_designer.config.analysis.dataset_profiler import DatasetProfilerResults


document = {
    "num_records": 1,
    "target_num_records": 1,
    "column_statistics": [
        {
            "column_type": "general",
            "column_name": "text",
            "num_records": 1,
            "num_null": 0,
            "num_unique": 1,
            "pyarrow_dtype": "string",
            "simple_dtype": "str",
        }
    ],
    "column_profiles": [
        {
            "column_name": "quality",
            "summaries": {
                "helpfulness": {
                    "score_name": "helpfulness",
                    "summary": "No observations",
                    "score_samples": [],
                }
            },
            "score_distributions": {
                "scores": {"helpfulness": []},
                "reasoning": {"helpfulness": []},
                "distribution_types": {"helpfulness": "categorical"},
                "distributions": {"helpfulness": "--"},
                "histograms": {
                    "helpfulness": {"categories": [], "counts": []},
                },
            },
        }
    ],
}

results = DatasetProfilerResults.model_validate(document)
results.to_report(Path("report.html"))

Model validation succeeds, then report rendering raises ValueError: max() arg is an empty sequence.

Expected behavior

An empty categorical histogram should render an empty-state placeholder, or it should be rejected during model validation with a clear validation error. to_report() should not raise an exception for an accepted input.

Agent Diagnostic / Prior Investigation

Reproduced on the current main branch. A histogram containing one category and one count renders successfully as a negative control.

CategoricalHistogramData in packages/data-designer-config/src/data_designer/config/analysis/column_statistics.py allows empty lists. The report path converts those lists into an empty mapping and passes it to create_rich_histogram_table() in packages/data-designer-config/src/data_designer/config/utils/visualization.py, which calls max(data.values()) without checking whether the mapping is empty.

The issue tracker was searched for CategoricalHistogramData, empty categorical histograms, and max() arg is an empty sequence; no matching issue was found.

Additional context

Regression coverage should exercise the public DatasetProfilerResults.to_report() method with empty categorical histogram data. Handling the empty state in the renderer would also make the shared visualization helper more robust.

Checklist

• I reproduced this issue or provided a minimal example
• I searched the docs/issues myself, or had my agent do so
• If I used an agent, I included its diagnostics above

Start with CategoricalHistogramData in packages/data-designer-config/src/data_designer/config/analysis/column_statistics.py, then follow DatasetProfilerResults.to_report() to create_rich_histogram_table() in packages/data-designer-config/src/data_designer/config/utils/visualization.py. Add regression coverage through the public to_report() entry point using the provided empty histogram data; done means accepted input no longer raises and the empty state is handled clearly.