BUGFIX: Render dropdown contents even when dropdown is closed

Until now, dropdown contents were `null`ed when the dropdown is closed.
This leads to unneccessary re-renders every time the dropdown visibility
is toggled.

This commit changes this logic. Dropdown contents are now always
rendered, but hidden when the dropdown is closed.

packages/react-ui-components/src/DropDown/__snapshots__/contents.spec.tsx.snap

@@ -0,0 +1,15 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`<ShallowDropDownContents/> should not render when having no children. 1`] = `
+<ul
+  aria-hidden="true"
+  aria-label="dropdown"
+  className="baseDropDownContentsClassName"
+  data-ignore_click_outside={true}
+  onClick={[MockFunction]}
+  role="listbox"
+  style={Object {}}
+>
+  Foo children
+</ul>
+`;

packages/react-ui-components/src/DropDown/contents.spec.tsx

@@ -21,7 +21,7 @@ describe('<ShallowDropDownContents/>', () => {
     it('should not render when having no children.', () => {
         const wrapper = shallow(<ShallowDropDownContents {...props}/>);
 
-        expect(toJson(wrapper)).toBeFalsy();
+        expect(toJson(wrapper)).toMatchSnapshot();
     });
 
     it('should allow the propagation of "className" with the "className" prop.', () => {

packages/react-ui-components/src/DropDown/contents.tsx

@@ -214,25 +214,22 @@ export default class ShallowDropDownContents extends PureComponent<ShallowDropDo
             }
         );
 
-        if (isOpen) {
-            const contents = (
-                <ul
-                    className={finalClassName}
-                    aria-hidden={isOpen ? 'false' : 'true'}
-                    aria-label="dropdown"
-                    role="listbox"
-                    onClick={closeDropDown}
-                    style={this.state.style}
-                    data-ignore_click_outside={true}
-                >
-                    {children}
-                </ul>
-            );
-
-            return scrollable
-                ? ReactDOM.createPortal(contents, document.body)
-                : contents;
-        }
-        return null;
+        const contents = (
+            <ul
+                className={finalClassName}
+                aria-hidden={isOpen ? 'false' : 'true'}
+                aria-label="dropdown"
+                role="listbox"
+                onClick={closeDropDown}
+                style={this.state.style}
+                data-ignore_click_outside={true}
+            >
+                {children}
+            </ul>
+        );
+
+        return scrollable
+            ? ReactDOM.createPortal(contents, document.body)
+            : contents;
     }
 }