BaseBufferBuilder.isNull returns the inverse of the null state

[maltzsama]

Describe the bug, including details regarding any error messages, version, and platform.

Description

BaseBufferBuilder.isNull(_:) reports a value as null when it is valid, and as valid when it is null.

Root Cause

The builders write a standard Arrow validity bitmap: append calls setBit for non-null values and clearBit for null values, so a set bit means valid. The function returns isSet, which means it reports the opposite of the intended result.

public func isNull(_ index: UInt) -> Bool {
    return self.nulls.length == 0 || BitUtility.isSet(index + self.offset, buffer: self.nulls)
}

This is confirmed by two other places in the codebase that treat the same bitmap correctly:

• ArrowData.isNull(_:) returns nullBuffer.length > 0 && !BitUtility.isSet(...)
• ListBufferBuilder.isNull(_:) returned !BitUtility.isSet(...) (redundant override)

Affected Builders

• FixedBufferBuilder
• BoolBufferBuilder
• VariableBufferBuilder
• StructBufferBuilder
• Date32BufferBuilder and Date64BufferBuilder (via AbstractWrapperBufferBuilder)

Impact

No code inside the library calls the builders' isNull — all internal readers go through ArrowData.isNull — so this affects external users of the public builder API only.