Fix eslint6 react/self-closing-comp

[MAILPOET-1140]

assets/js/src/help-tooltip.jsx

@@ -32,8 +32,7 @@ function Tooltip(props) {
         data-event="click"
         data-tip
         data-for={tooltipId}
-      >
-      </span>
+       />
       <ReactTooltip
         globalEventOff="click"
         multiline={true}

assets/js/src/listing/header.jsx

@@ -72,7 +72,7 @@ const ListingColumn = React.createClass({
       label = (
         <a onClick={this.handleSort}>
           <span>{ this.props.column.label }</span>
-          <span className="sorting-indicator"></span>
+          <span className="sorting-indicator" />
         </a>
       );
     } else {

assets/js/src/newsletters/listings/mixins.jsx

@@ -129,7 +129,7 @@ const QueueMixin = {
           <span
             className="mailpoet_progress_bar"
             style={{ width: `${progressBarWidth}%` }}
-              ></span>
+               />
           <span className="mailpoet_progress_label">
             { percentage }
           </span>

assets/js/src/newsletters/templates/template_box.jsx

@@ -107,7 +107,7 @@ class TemplateBox extends React.Component {
       preview = (
         <a href="javascript:;" onClick={this.onPreview}>
           <img src={thumbnail} />
-          <div className="mailpoet_overlay"></div>
+          <div className="mailpoet_overlay" />
         </a>
       );
     }