fix: add test for empty path

Author: anshumanv

test/__snapshots__/validate-plugin-options.test.js.snap

@@ -8,6 +8,11 @@ exports[`validate options should throw an error on the "attributes" option with
    -> Read more at https://github.com/webpack-contrib/mini-css-extract-plugin#attributes"
 `;
 
+exports[`validate options should throw an error on the "chunkFilename" option with "" value 1`] = `
+"Invalid options object. Mini CSS Extract Plugin has been initialized using an options object that does not match the API schema.
+ - options.chunkFilename should be a non-empty string."
+`;
+
 exports[`validate options should throw an error on the "chunkFilename" option with "/styles/[id].css" value 1`] = `
 "Invalid options object. Mini CSS Extract Plugin has been initialized using an options object that does not match the API schema.
  - options.chunkFilename: A relative path is expected. However, the provided value \\"/styles/[id].css\\" is an absolute path!"
@@ -24,6 +29,11 @@ exports[`validate options should throw an error on the "chunkFilename" option wi
     * options.chunkFilename should be an instance of function."
 `;
 
+exports[`validate options should throw an error on the "filename" option with "" value 1`] = `
+"Invalid options object. Mini CSS Extract Plugin has been initialized using an options object that does not match the API schema.
+ - options.filename should be a non-empty string."
+`;
+
 exports[`validate options should throw an error on the "filename" option with "/styles/[name].css" value 1`] = `
 "Invalid options object. Mini CSS Extract Plugin has been initialized using an options object that does not match the API schema.
  - options.filename: A relative path is expected. However, the provided value \\"/styles/[name].css\\" is an absolute path!"

test/validate-plugin-options.test.js

@@ -7,11 +7,11 @@ describe("validate options", () => {
         "[name].css",
         ({ name }) => `${name.replace("/js/", "/css/")}.css`,
       ],
-      failure: [true, "/styles/[name].css"],
+      failure: [true, "/styles/[name].css", ""],
     },
     chunkFilename: {
       success: ["[id].css", ({ chunk }) => `${chunk.id}.${chunk.name}.css`],
-      failure: [true, "/styles/[id].css"],
+      failure: [true, "/styles/[id].css", ""],
     },
     ignoreOrder: {
       success: [true, false],