Fix gradient dropping optional arguments set to 0

[11bit]

Follow-up to #88, which introduced progressive_blur with explicit undefined checks rather than the truthiness checks gradient uses. This brings gradient in line — and it turns out to be a real bug fix, not just a style change.

The bug

color, direction, start and stop were guarded with if (value) and rendered with value || "", so any falsy value was treated as "not provided":

Input	Before	After
{ opacity: 0.5, stop: 0 }	gr:0.5 — imgproxy applies the default stop of 1.0	gr:0.5::::0
{ opacity: 0.5, start: 0 }	gr:0.5	gr:0.5:::0
{ opacity: 0.5, direction: 0 }	gr:0.5	gr:0.5::0

The stop: 0 case is the damaging one: the caller asks for a gradient that stops immediately and gets one spanning the whole image instead. start: 0 and direction: 0 happen to coincide with imgproxy's defaults, so those were harmless in effect but still dropped the caller's input.

Falsy values were also skipping validation entirely — start: null from a vanilla-JS caller rendered as an empty argument instead of raising.

The fix

if (value !== undefined) for validation, value ?? "" for rendering. Nothing else changes: omitted arguments still render empty and trailing : are still trimmed.

Behavior change worth noting

{ opacity: 0.5, color: "" } now throws gradient.color must be 3, 6 or 8 characters long (with alpha) instead of silently rendering an empty argument. An empty string was never a valid hex color, so this surfaces a caller bug rather than hiding it — but it is a new exception on input that previously "worked", hence the patch changeset.

Verification

• tsc --noEmit clean, eslint src tests clean
• vitest run → 97 files / 1100 tests passing (5 new, covering stop: 0, start: 0, direction: 0, start: null and color: "")
• No existing test needed changing

Out of scope

The same truthiness pattern appears in resize, trim, size, watermark, watermarkSize, unsharpMasking and pngOptions. I have not audited whether any of them drops a meaningful 0 the way gradient did — worth a separate look.

🤖 Generated with Claude Code

https://claude.ai/code/session_01G8Aa3xeVUCaxKtQJDzZhMo