Prevent non empty unions and clarify null docs

[connortsui20]

Rationale for this change

Tracking issue: #8769

What changes are included in this PR?

As discussed offline, we have decided to disallow empty unions because we do not think there is a good usecase for it. And as you can see in the diffs here, that meant we could not choose a good default for the "zero value" of scalars.

This PR prevents empty unions, as well as clarifies some of the null docs around the union scalar accessors.

What APIs are changed? Are there any user-facing changes?

Some name changes to make things more obvious

[github-actions]

Polar Signals Profiling Results

Latest Run

Status	Commit	Job	Attempt	Link
🟢 Done	75d61fd		1	Explore Profiling Data

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[github-actions]

Benchmarks: Vortex queries 📖

Verdict: No clear signal (low confidence)
Attributed Vortex impact: +4.6%
Engines: DataFusion No clear signal (+7.6%, low confidence) · DuckDB No clear signal (+1.8%, low confidence)
Vortex (geomean): 1.009x ➖
Parquet (geomean): 0.986x ➖
Shifts: Parquet (control) -1.4% · Median polish +1.0%

How to read Verdict and Engines

• Verdict: Overall PR-level signal after subtracting baseline drift estimated from Parquet control rows. It can be Likely improvement, Likely regression, or No clear signal.
• Engines: Per-engine attribution. DataFusion is compared against DataFusion/Parquet controls; DuckDB is compared against DuckDB/Parquet controls. This answers whether each engine improved or regressed independently.
• Confidence: Based on directional consistency, share of rows above the noise floor, and control-run noise.

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datafusion / vortex-file-compressed (1.045x ➖, 0↑ 0↓)

name	PR 75d61fd (ns)	base 62af4a1 (ns)	ratio (PR/base)
vortex_q00/datafusion:vortex-file-compressed	9795641	9464067	1.04
vortex_q01/datafusion:vortex-file-compressed	6255843	5931343	1.05

datafusion / parquet (0.971x ➖, 0↑ 0↓)

name	PR 75d61fd (ns)	base 62af4a1 (ns)	ratio (PR/base)
vortex_q00/datafusion:parquet	19999395	20451849	0.98
vortex_q01/datafusion:parquet	4475686	4637790	0.97

duckdb / vortex-file-compressed (1.019x ➖, 0↑ 0↓)

name	PR 75d61fd (ns)	base 62af4a1 (ns)	ratio (PR/base)
vortex_q00/duckdb:vortex-file-compressed	10021423	9877811	1.01
vortex_q01/duckdb:vortex-file-compressed	6058631	5919041	1.02

duckdb / parquet (1.001x ➖, 0↑ 0↓)

name	PR 75d61fd (ns)	base 62af4a1 (ns)	ratio (PR/base)
vortex_q00/duckdb:parquet	23289169	23193585	1.00
vortex_q01/duckdb:parquet	9359157	9377576	1.00

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No file size changes detected.