Don't assume a frontend cache section exists in the env.php file.

Author: hostep

lib/internal/Magento/Framework/App/Cache/Frontend/Pool.php

@@ -86,7 +86,7 @@ protected function _getCacheSettings()
          * default cache_dir setting from di.xml when a cache id_prefix is configured in app/etc/env.php.
          */
         $cacheInfo = $this->deploymentConfig->getConfigData(FrontendPool::KEY_CACHE);
-        if (null !== $cacheInfo) {
+        if (null !== $cacheInfo && array_key_exists(FrontendPool::KEY_FRONTEND_CACHE, $cacheInfo)) {
             return array_replace_recursive($this->_frontendSettings, $cacheInfo[FrontendPool::KEY_FRONTEND_CACHE]);
         }
         return $this->_frontendSettings;

lib/internal/Magento/Framework/App/Test/Unit/Cache/Frontend/PoolTest.php

@@ -123,6 +123,11 @@ public function initializationParamsDataProvider(): array
                 [Pool::DEFAULT_FRONTEND_ID => ['default_option' => 'default_value']],
                 ['default_option' => 'default_value']
             ],
+            'deployment config, default settings but no frontend cache' => [
+                [],
+                [Pool::DEFAULT_FRONTEND_ID => ['default_option' => 'default_value']],
+                ['default_option' => 'default_value']
+            ],
             'deployment config, default settings' => [
                 ['frontend' => [Pool::DEFAULT_FRONTEND_ID => ['configured_option' => 'configured_value']]],
                 [Pool::DEFAULT_FRONTEND_ID => ['default_option' => 'default_value']],