fix: Make hot reload work on files containing only import statements

If a file only has import statements, the parser would crash when hot
module reloading was enabled because firstNonImportDeclarationNode would
be undefined. This makes those files work by adding the hot module block
at the end of the file in those cases.

src/index.js

@@ -97,7 +97,11 @@ export default ({
 
     const hotAcceptStatement = t.ifStatement(test, consequent);
 
-    firstNonImportDeclarationNode.insertBefore(hotAcceptStatement);
+    if (firstNonImportDeclarationNode) {
+      firstNonImportDeclarationNode.insertBefore(hotAcceptStatement);
+    } else {
+      programPath.pushContainer('body', hotAcceptStatement);
+    }
   };
 
   const getTargetResourcePath = (path: Object, stats:Object) => {

test/fixtures/react-css-modules/adds module hot accept for CSS imports when file only contains import statements/actual.js

@@ -0,0 +1 @@
+import './bar.css';

test/fixtures/react-css-modules/adds module hot accept for CSS imports when file only contains import statements/bar.css

@@ -0,0 +1 @@
+.a {}

test/fixtures/react-css-modules/adds module hot accept for CSS imports when file only contains import statements/expected.js

@@ -0,0 +1,7 @@
+import './bar.css';
+
+if (module.hot) {
+  module.hot.accept('./bar.css', function () {
+    require('./bar.css');
+  });
+}

test/fixtures/react-css-modules/adds module hot accept for CSS imports when file only contains import statements/options.json

@@ -0,0 +1,11 @@
+{
+  "plugins": [
+    [
+      "../../../../src",
+      {
+        "generateScopedName": "[name]__[local]",
+        "webpackHotModuleReloading": true
+      }
+    ]
+  ]
+}