Allow form fields styling

[PREMIUM-44]

assets/js/src/form/fields/field.jsx

@@ -111,7 +111,7 @@ define([
       }
 
       return (
-        <tr>
+        <tr className={`form-field-row-${this.props.field.name}`}>
           <th scope="row">
             <label
               htmlFor={`field_${this.props.field.name}`}

assets/js/src/form/fields/selection.jsx

@@ -164,6 +164,7 @@ define([
       // we must have a blank <option> as the first option in the <select> control.
       if (this.allowMultipleValues()) return undefined;
       if (this.props.field.placeholder) return (<option />);
+      return undefined;
     },
     render: function () {
       const options = this.state.items.map((item, index) => {