Fix null source content ignoring on processing source map

src/index.js

@@ -94,7 +94,9 @@ export default async function loader(input, inputMap) {
       let sourceContent;
 
       const originalSourceContent =
-        map.sourcesContent && typeof map.sourcesContent[i] !== "undefined"
+        map.sourcesContent &&
+        typeof map.sourcesContent[i] !== "undefined" &&
+        map.sourcesContent[i] !== null
           ? map.sourcesContent[i]
           : // eslint-disable-next-line no-undefined
             undefined;

test/__snapshots__/loader.test.js.snap

@@ -334,6 +334,32 @@ Object {
 
 exports[`source-map-loader should process inlined sources: warnings 1`] = `Array []`;
 
+exports[`source-map-loader should process null in sources content: css 1`] = `
+"with SourceMap
+// comment
+"
+`;
+
+exports[`source-map-loader should process null in sources content: errors 1`] = `Array []`;
+
+exports[`source-map-loader should process null in sources content: map 1`] = `
+Object {
+  "file": "null-in-sources-content.js",
+  "mappings": "AAAA",
+  "sources": Array [
+    "/test/fixtures/null-in-sources-content.txt - (normalized for test)",
+  ],
+  "sourcesContent": Array [
+    "with SourceMap
+// comment
+",
+  ],
+  "version": 3,
+}
+`;
+
+exports[`source-map-loader should process null in sources content: warnings 1`] = `Array []`;
+
 exports[`source-map-loader should process percent-encoding path: css 1`] = `
 "with SourceMap
 // comment

test/fixtures/null-in-sources-content.js

@@ -0,0 +1,3 @@
+with SourceMap
+// #sourceMappingURL=null-in-sources-content.js.map
+// comment

test/fixtures/null-in-sources-content.txt

@@ -0,0 +1,2 @@
+with SourceMap
+// comment

test/loader.test.js

@@ -107,6 +107,27 @@ describe("source-map-loader", () => {
     expect(getErrors(stats)).toMatchSnapshot("errors");
   });
 
+  it("should process null in sources content", async () => {
+    const testId = "null-in-sources-content.js";
+    const compiler = getCompiler(testId);
+    const stats = await compile(compiler);
+    const codeFromBundle = getCodeFromBundle(stats, compiler);
+    const deps = stats.compilation.fileDependencies;
+
+    const dependencies = [
+      path.resolve(__dirname, "fixtures", "null-in-sources-content.js.map"),
+    ];
+
+    dependencies.forEach((fixture) => {
+      expect(deps.has(fixture)).toBe(true);
+    });
+    expect(codeFromBundle.map).toBeDefined();
+    expect(normalizeMap(codeFromBundle.map)).toMatchSnapshot("map");
+    expect(codeFromBundle.css).toMatchSnapshot("css");
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+  });
+
   it("should reject http SourceMaps", async () => {
     const testId = "http-source-map.js";
     const compiler = getCompiler(testId);