add check if pages-manifest exists before reading (netlify#147)

Author: lindsaylevine

lib/helpers/getPagesManifest.js

@@ -1,11 +1,11 @@
 const { join } = require("path");
-const { readJSONSync } = require("fs-extra");
+const { existsSync, readJSONSync } = require("fs-extra");
 const { NEXT_DIST_DIR } = require("../config");
 
 const getPagesManifest = () => {
- const contents = readJSONSync(
-  join(NEXT_DIST_DIR, "serverless", "pages-manifest.json")
- );
+ const manifestPath = join(NEXT_DIST_DIR, "serverless", "pages-manifest.json");
+ if (!existsSync(manifestPath)) return {};
+ const contents = readJSONSync(manifestPath);
  // Next.js mistakenly puts backslashes in certain paths on Windows, replace
  Object.entries(contents).forEach(([key, value]) => {
  contents[key] = value.replace(/\\/g, "/");