do not hide icons if text is an empty string

Empty text strings do not have any collision boxes so the collision
index check was never done and the text was not counted as "placed".
This fixes that by making "placed" the default when the feature has
text.

fix #6160

src/symbol/placement.js

@@ -118,8 +118,8 @@ class Placement {
         for (const symbolInstance of bucket.symbolInstances) {
             if (!seenCrossTileIDs[symbolInstance.crossTileID]) {
 
-                let placeText = false;
-                let placeIcon = false;
+                let placeText = symbolInstance.feature.text !== undefined;
+                let placeIcon = symbolInstance.feature.icon !== undefined;
                 let offscreen = true;
 
                 let placedGlyphBoxes = null;

test/integration/render-tests/regressions/mapbox-gl-js#6160/style.json

@@ -0,0 +1,73 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "width": 64,
+      "height": 64,
+      "description": "Checks that icons are not hidden when the text is an empty string"
+    }
+  },
+  "glyphs": "local://glyphs/{fontstack}/{range}.pbf",
+  "sources": {
+    "geojson": {
+      "type": "geojson",
+      "data": {
+        "type": "FeatureCollection",
+        "features": [
+            {
+                "type": "Feature",
+                "geometry": {
+                    "type": "Point",
+                    "coordinates": [
+                      0,
+                      0
+                    ]
+                },
+                "properties": {
+                    "text": "OK"
+                }
+            },
+            {
+                "type": "Feature",
+                "geometry": {
+                    "type": "Point",
+                    "coordinates": [
+                      20,
+                      0
+                    ]
+                },
+                "properties": {
+                    "text": ""
+                }
+            }
+        ]
+      }
+    }
+  },
+  "sprite": "local://sprites/sprite",
+  "layers": [
+    {
+      "id": "symbol",
+      "type": "symbol",
+      "source": "geojson",
+      "layout": {
+        "symbol-placement": "point",
+        "text-allow-overlap": true,
+        "icon-allow-overlap": true,
+        "icon-image": "triangle-12",
+        "text-field": "{text}",
+        "text-font": [
+          "Open Sans Semibold",
+          "Arial Unicode MS Bold"
+        ]
+      },
+      "paint": {
+        "icon-opacity": 0.5,
+        "text-translate": [
+          -10,
+          0
+        ]
+      }
+    }
+  ]
+}