[FIX] web: display buttons without icon in list view

Previously, the button was completely invisible when no icon
was given, which is a regression from saas-15 where the string
attribute is used to write the button label.

addons/web/static/src/js/views/list/list_renderer.js

@@ -271,10 +271,14 @@ var ListRenderer = BasicRenderer.extend({
     _renderButton: function (record, node) {
         var $button = $('<button>', {
             type: 'button',
-            class: 'o_icon_button',
             title: node.attrs.string,
         });
-        $button.append($('<i>', {class: 'fa ' + node.attrs.icon}));
+        if (node.attrs.icon) {
+            $button.addClass('o_icon_button');
+            $button.append($('<i>', {class: 'fa ' + node.attrs.icon}));
+        } else {
+            $button.text(node.attrs.string);
+        }
 
         this._registerModifiers(node, record, $button);
 

addons/web/static/tests/views/list_tests.js

@@ -1046,6 +1046,24 @@ QUnit.module('Views', {
         list.destroy();
     });
 
+    QUnit.test('list view with a button without icon', function (assert) {
+        assert.expect(1);
+
+        var list = createView({
+            View: ListView,
+            model: 'foo',
+            data: this.data,
+            arch: '<tree string="Phonecalls" editable="top">' +
+                    '<field name="foo"/>' +
+                    '<button string="abc" type="object" name="schedule_another_phonecall"/>' +
+                '</tree>',
+        });
+
+        assert.strictEqual(list.$('table button').first().text(), 'abc',
+            "should have rendered a button with string attribute as label");
+        list.destroy();
+    });
+
     QUnit.test('list view, editable, can discard', function (assert) {
         assert.expect(5);