Return empty array if sections key is empty

The `sections` key may not be set during module installation or upgrade and
would result in an undefined index notice.

Author: pmclain

app/code/Magento/Config/Model/Config/Structure.php

@@ -281,6 +281,10 @@ protected function _createEmptyElement(array $pathParts)
     public function getFieldPathsByAttribute($attributeName, $attributeValue)
     {
         $result = [];
+        if (empty($this->_data['sections'])) {
+            return $result;
+        }
+
         foreach ($this->_data['sections'] as $section) {
             if (!isset($section['children'])) {
                 continue;

app/code/Magento/EncryptionKey/Setup/Patch/Data/SodiumChachaPatch.php

@@ -83,15 +83,10 @@ private function reEncryptSystemConfigurationValues()
 
         $this->scope->setCurrentScope(\Magento\Framework\App\Area::AREA_ADMINHTML);
 
-        try {
-            $paths = $this->structure->getFieldPathsByAttribute(
-                'backend_model',
-                \Magento\Config\Model\Config\Backend\Encrypted::class
-            );
-        } catch (\Exception $e) {
-            // This is thrown during initial application installation
-            return;
-        }
+        $paths = $this->structure->getFieldPathsByAttribute(
+            'backend_model',
+            \Magento\Config\Model\Config\Backend\Encrypted::class
+        );
 
         $this->scope->setCurrentScope($currentScope);