TimeSeries: Stacked bars empty tooltip for null value

<details><summary>stacked-bars-zero-line-width.json</summary> ```json { "annotations": { "list": [ { "builtIn": 1, "datasource": { "type": "grafana", "uid": "-- Grafana --" }, "enable": true, "hide": true, "iconColor": "rgba(0, 211, 255, 1)", "name": "Annotations & Alerts", "type": "dashboard" } ] }, "editable": true, "fiscalYearStartMonth": 0, "graphTooltip": 0, "id": 21475, "links": [], "panels": [ { "datasource": { "type": "grafana-testdata-datasource", "uid": "PD8C576611E62080A" }, "fieldConfig": { "defaults": { "color": { "mode": "palette-classic" }, "custom": { "axisBorderShow": false, "axisCenteredZero": false, "axisColorMode": "text", "axisLabel": "", "axisPlacement": "auto", "barAlignment": 0, "barWidthFactor": 0.6, "drawStyle": "bars", "fillOpacity": 100, "gradientMode": "none", "hideFrom": { "legend": false, "tooltip": false, "viz": false }, "insertNulls": false, "lineInterpolation": "linear", "lineWidth": 0, "pointSize": 5, "scaleDistribution": { "type": "linear" }, "showPoints": "auto", "showValues": false, "spanNulls": false, "stacking": { "group": "A", "mode": "normal" }, "thresholdsStyle": { "mode": "off" } }, "mappings": [], "thresholds": { "mode": "absolute", "steps": [ { "color": "green", "value": 0 }, { "color"…