Guard against zero/NaN tick in _lines() when container has zero dimensions

[HughF]

Problem

When a Leaflet map is initialised inside a hidden container (e.g. a jQuery UI tab, Bootstrap tab, or modal), map.getSize() returns {x:0, y:0}. This causes _tickRange() to compute tick = 0 or NaN, which in turn makes _lines() return an unbounded or degenerate line array — resulting in no grid being rendered, and in some cases a hang or excessive paint time.

Fix

Add an early return in _lines() before the loop:

if (!tick || !isFinite(tick)) { return []; }

This is a no-op for normal usage. It only fires when ticks is 0 or the bounds produce a degenerate range, making the function safe to call regardless of container visibility.

Reproducer

Initialise an L.Grid layer on a map inside a hidden jQuery UI tab. Open the tab (triggering invalidateSize()). Without this fix, the grid does not render. With it, the grid renders correctly once the container is visible.