test(loader): Add test for the case when publicPath is an empty string

test/cases/publicpath-emptystring/expected/main.css

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+body { background: red; background-image: url(cd0bb358c45b584743d8ce4991777c42.svg); }
+

test/cases/publicpath-emptystring/index.js

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+import './style.css';

test/cases/publicpath-emptystring/style.css

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+body { background: red; background-image: url(./react.svg); }

test/cases/publicpath-emptystring/webpack.config.js

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+import Self from '../../../src';
+
+module.exports = {
+  entry: './index.js',
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          {
+            loader: Self.loader,
+            options: {
+              publicPath: '',
+            },
+          },
+          'css-loader',
+        ],
+      },
+      {
+        test: /\.(svg|png)$/,
+        use: [
+          {
+            loader: 'file-loader',
+            options: {
+              filename: '[name].[ext]',
+            },
+          },
+        ],
+      },
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};