omit cache key entirely when build cache is disabled

It appears that cache: false is different than omitting the cache key
entirely. Using cache: false appears to even disable in-memory caching
that Webpack does when rebuilding in "watch" mode or for the dev-server

Author: weaverryan

lib/config-generator.js

@@ -67,11 +67,14 @@ class ConfigGenerator {
             },
             plugins: this.buildPluginsConfig(),
             optimization: this.buildOptimizationConfig(),
-            cache: this.buildCacheConfig(),
             watchOptions: this.buildWatchOptionsConfig(),
             devtool: false,
         };
 
+        if (this.webpackConfig.usePersistentCache) {
+            config.cache = this.buildCacheConfig();
+        }
+
         if (this.webpackConfig.useSourceMaps) {
             if (this.webpackConfig.isProduction()) {
                 // https://webpack.js.org/configuration/devtool/#for-production
@@ -511,10 +514,6 @@ class ConfigGenerator {
     }
 
     buildCacheConfig() {
-        if (!this.webpackConfig.usePersistentCache) {
-            return false;
-        }
-
         const cache = {};
 
         cache.type = 'filesystem';