fix(components/base_component): Components without an init() function are not renderable

Author: kantord

src/components/base_component.js

@@ -75,7 +75,7 @@ const render_component = (args, instance_args, ...rest) =>
 const create_element = (init_func, instance_args, selection) => {
   try {
     return (typeof init_func === 'function')
-      ? init_func(instance_args, selection) : null 
+      ? init_func(instance_args, selection) : selection.append('span')
   } catch(error) {
     selection.call(show_error_message(error))
   }

src/components/base_component.test.js

@@ -170,6 +170,12 @@ describe('Component', function() {
     my_render.should.be.calledWith({}, my_selection, [1])
   })
 
+  it('if there is no init() function, a <span> is created', () => {
+    const { my_init } = call_test_component_with(
+      {'instance_args': {}, 'has_init': false})
+    d3.selectAll('span').size().should.equal(1)
+  })
+
   it('if there is an init() function, it should be called', () => {
     const { my_init } = call_test_component_with(
       {'instance_args': {}, 'has_init': true})