Flowmap links ignore points on plot edges

Expected behaviour

A flowmap link whose referenced point has plotX = 0 or plotY = 0 should resolve that endpoint normally.

Actual behaviour

The linked-point resolver checks both coordinates by truthiness. A valid point on the left or top plot edge is therefore treated as unresolved and the flow endpoint remains undefined.

Live demo with steps to reproduce

1. Create a map chart with a mappoint series and a flowmap referencing two point IDs.
2. Place one referenced point at plotX = 0 or plotY = 0.
3. Translate the flowmap series.
4. Observe that fromPos/toPos is not populated for that endpoint.

Product version

Current main