fix: do not crash if pkg.(d|devD)ependencies unset (#667)

Author: mcous

src/index.js

@@ -162,7 +162,9 @@ export default async function loader(content, sourceMap, meta) {
           }
 
           if (pkg) {
-            if (!pkg.dependencies.postcss && !pkg.devDependencies.postcss) {
+            const { dependencies = {}, devDependencies = {} } = pkg;
+
+            if (!dependencies.postcss && !devDependencies.postcss) {
               this.emitWarning(
                 new Error(
                   "Add postcss as project dependency. postcss is not a peer dependency for postcss-loader. " +

test/fixtures/package-json-files/no-postcss/package.json

@@ -1,4 +1 @@
-{
-  "dependencies": {},
-  "devDependencies": {}
-}
+{}

test/fixtures/package-json-files/postcss-v8-in-dependencies/package.json

@@ -1,6 +1,5 @@
 {
   "dependencies": {
     "postcss": "^8.0.0"
-  },
-  "devDependencies": {}
-}
+  }
+}

test/fixtures/package-json-files/postcss-v8-in-devDependencies/package.json

@@ -1,6 +1,5 @@
 {
-  "dependencies": {},
   "devDependencies": {
     "postcss": "^8.0.0"
   }
-}
+}