Fix buggy image coercion so image names resolve correctly (#8839)

src/style-spec/expression/definitions/coercion.js

@@ -102,8 +102,8 @@ class Coercion implements Expression {
             // created by properties that expect the 'formatted' type.
             return Formatted.fromString(valueToString(this.args[0].evaluate(ctx)));
         } else if (this.type.kind === 'image') {
-            const name = valueToString(this.args[0].evaluate(ctx));
-            return ResolvedImage.fromString({name, available: false});
+            const name = this.args[0].evaluate(ctx);
+            return typeof name === 'string' ? ResolvedImage.fromString({name, available: false}) : name;
         } else {
             return valueToString(this.args[0].evaluate(ctx));
         }

test/integration/render-tests/icon-image/image-expression/style.json

@@ -0,0 +1,32 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "height": 64,
+      "width": 64
+    }
+  },
+  "sources": {
+    "geojson": {
+      "type": "geojson",
+      "data": {
+        "type": "Point",
+        "coordinates": [
+          0,
+          0
+        ]
+      }
+    }
+  },
+  "sprite": "local://sprites/sprite",
+  "layers": [
+    {
+      "id": "text",
+      "type": "symbol",
+      "source": "geojson",
+      "layout": {
+        "icon-image": ["coalesce", ["image", "dot.sdf"], ["image", "foo"]]
+      }
+    }
+  ]
+}