[keichi]

Problem

Validation in string(), buffer(), wrapped() and array() checks length/lengthInBytes by truthiness, so a literal 0 is treated as missing.

Repro

new Parser().buffer("b", { length: 0 });
// Error: length or readUntil must be defined for buffer.

Same for string("s", { length: 0 }) and array("a", { length: 0, type: "uint8" }).

Fix

• Use == null checks for length/lengthInBytes in the validations (including the greedy mutual-exclusion and stripNull checks).
• generateString used truthiness to pick the length branch; switched to != null.
• For length + zeroTerminated, check the length bound before reading a byte, so length 0 no longer consumes one byte (behavior for length >= 1 is unchanged).
• generateArray already compared against undefined, so length 0 arrays work once validation passes.

Added regression tests for length 0 strings (plain, zeroTerminated, stripNull), buffers and arrays (length and lengthInBytes).