Revert string concat optimizations - there is no real improvement

I've run performance tests for string concat vs. array join:
https://gist.github.com/andrey-legayev/24ed101a34d4775bf1b6f9879581f51a

No real performance improvement (which surprised me).
But anyway - I'm rolling back string concatenation improvements, because it doesn't make any sense.

Author: andrey-legayev

lib/internal/Magento/Framework/View/Element/AbstractBlock.php

@@ -520,14 +520,9 @@ public function getChildHtml($alias = '', $useCache = true)
                 $out = $layout->renderElement($childName, $useCache);
             }
         } else {
-            $outParts = [];
             foreach ($layout->getChildNames($name) as $child) {
-                $elementHtml = $layout->renderElement($child, $useCache);
-                if (!empty($elementHtml)) {
-                    $outParts[] = $elementHtml;
-                }
+                $out .= $layout->renderElement($child, $useCache);
             }
-            $out = join('', $outParts);
         }
 
         return $out;
@@ -556,14 +551,9 @@ public function getChildChildHtml($alias, $childChildAlias = '', $useCache = tru
             $childChildName = $layout->getChildName($childName, $childChildAlias);
             $out = $layout->renderElement($childChildName, $useCache);
         } else {
-            $outParts = [];
             foreach ($layout->getChildNames($childName) as $childChild) {
-                $elementHtml = $layout->renderElement($childChild, $useCache);
-                if (!empty($elementHtml)) {
-                    $outParts[] = $elementHtml;
-                }
+                $out .= $layout->renderElement($childChild, $useCache);
             }
-            $out = join('', $outParts);
         }
         return $out;
     }

lib/internal/Magento/Framework/View/Layout.php

@@ -586,16 +586,13 @@ protected function _renderUiComponent($name)
      */
     protected function _renderContainer($name, $useCache = true)
     {
-        $htmlParts = [];
+        $html = '';
         $children = $this->getChildNames($name);
         foreach ($children as $child) {
-            $childHtml = $this->renderElement($child, $useCache);
-            if (!empty($childHtml)) {
-                $htmlParts[] = $childHtml;
-            }
+            $html .= $this->renderElement($child, $useCache);
         }
-        if (empty($htmlParts) || !$this->structure->getAttribute($name, Element::CONTAINER_OPT_HTML_TAG)) {
-            return join('', $htmlParts);
+        if ($html == '' || !$this->structure->getAttribute($name, Element::CONTAINER_OPT_HTML_TAG)) {
+            return $html;
         }
 
         $htmlId = $this->structure->getAttribute($name, Element::CONTAINER_OPT_HTML_ID);
@@ -609,7 +606,8 @@ protected function _renderContainer($name, $useCache = true)
         }
 
         $htmlTag = $this->structure->getAttribute($name, Element::CONTAINER_OPT_HTML_TAG);
-        $html = sprintf('<%1$s%2$s%3$s>%4$s</%1$s>', $htmlTag, $htmlId, $htmlClass, join('', $htmlParts));
+
+        $html = sprintf('<%1$s%2$s%3$s>%4$s</%1$s>', $htmlTag, $htmlId, $htmlClass, $html);
 
         return $html;
     }