fix: handle empty data array in HeaderViz sparkline

[oliviabrn]

Background

The HeaderViz component renders small inline SVGs (sparklines, mini bars, donuts, metric capsules) in page headers. The renderSparkline function computes step = width / (points.length - 1), which produces Infinity when the data array has exactly one element, leading to invalid SVG path coordinates.

Changes

Added an early return guard at the top of renderSparkline:

if (points.length < 2) return null

A sparkline needs at least two points to draw a line segment. The existing defaultData.length === 0 check at line 171 already handles the empty case, but it did not cover single-element arrays. The other render functions (renderMiniBars, renderDonut, renderMetricCapsules) work fine with a single data point, so they do not need this guard.

Notes

• No functional change for arrays with 2+ elements.
• renderMiniBars with a single value still renders a valid single bar.
• The component already returns null for empty arrays.

Verified by reviewing the diff and confirming the math: with points.length === 1, step would be 120 / 0 = Infinity, and x = 0 * Infinity = NaN, producing invalid SVG path data.