fix: unclear 'show no data' (#573)

Fix: https://dhis2.atlassian.net/browse/DHIS2-15799
Replace empty string by 'No data' in hoverLabel when value is missing.
Handle tranlsation of 'No data' string.
Update associated test.

jest.stub.js

@@ -6,6 +6,7 @@ global.mockMapGL = {
     getLayer: jest.fn(),
     getSource: jest.fn(),
     setFeatureState: jest.fn(),
+    _getUIString: jest.fn(),
 }
 
 global.mockMap = {

src/controls/controlsLocale.js

@@ -1,6 +1,7 @@
 const controlsLocale = {
     'SearchControl.SearchForPlace': 'Search for place or address',
     'FitBoundsControl.ZoomToContent': 'Zoom to content',
+    'HoverLabel.NoData': 'No data',
     'MeasureControl.MeasureDistancesAndAreas': 'Measure distances and areas',
     'MeasureControl.ClickStartMeasurement':
         'Click where you want to start the measurement',

src/layers/Layer.js

@@ -36,6 +36,8 @@ class Layer extends Evented {
         const layers = this.getLayers()
         const beforeId = map.getBeforeLayerId()
 
+        this.locale = mapgl._getUIString.bind(mapgl)
+
         if (images) {
             try {
                 await addImages(mapgl, images)
@@ -343,7 +345,9 @@ class Layer extends Evented {
             const { properties } = feature
             const content = (hoverLabel || label).replace(
                 /\{ *([\w_-]+) *\}/g,
-                (str, key) => properties[key] || ''
+                (str, key) =>
+                    properties[key] ||
+                    (key === 'value' ? this.locale('HoverLabel.NoData') : '')
             )
 
             this._map.showLabel(content, evt.lngLat)