Use `null_value` as implicit default when adding/defining nullable columns

[Jacc4224]

Background

When adding a new column with add_column(), a default value is always required. Similarly, when defining a schema for a new CTable, columns with a null_value sentinel still need a separate default. This is redundant: if a column is nullable, the most natural default for existing rows is the null sentinel itself.

Proposed behaviour

• If a spec declares null_value=X and no explicit default is provided, use X as the default automatically.
• If an explicit default is provided, it takes precedence as today.
• Same logic should apply at CTable construction time (when columns are pre-filled).

Example

import blosc2 as b2
from dataclasses import dataclass

@dataclass
class Row:
    temperature: float = b2.field(b2.float32(null_value=-9999.0))
    label: str = b2.field(b2.string(null_value=""))

t = b2.CTable(Row)
t.extend([{"temperature": 22.5, "label": "sunny"}] * 10)

# Current behaviour: must repeat the null sentinel
t.add_column("pressure", b2.float32(null_value=-9999.0), default=-9999.0)

# Proposed: null_value is picked up automatically
t.add_column("pressure", b2.float32(null_value=-9999.0))

# All existing rows should now read as null
assert t["pressure"].null_count() == 10

Open question

Should default become keyword-only and optional (default=None meaning "use null_value if available, else raise")? Or keep it positional but allow omitting it when null_value is set? Worth discussing before implementing.

[FrancescAlted]

Fixed by making CTable.add_column() derive the column default from the schema/field descriptor instead of accepting a separate explicit default argument.

The new API is:

t.add_column("weight", blosc2.field(blosc2.float64(), default=5.5))

For empty tables, a bare schema spec is still allowed:

  t.add_column("weight", blosc2.float64())

For non-empty tables, a default is required so existing live rows can be backfilled; otherwise add_column() raises a ValueError.

This keeps add_column() consistent with normal CTable schema definition, where defaults belong to the field/spec declaration rather than being a separate API parameter.

[FrancescAlted]

Commit 0d4a7f9 is also relevant here.