Fix prettier check in empty files (denoland/deno_std#302)

Original: e5a493e9e8

prettier/main.ts

@@ -107,8 +107,8 @@ async function checkFile(
   const text = await readFileIfExists(filename);
 
   if (!text) {
-    // The file is deleted. Skip.
-    return;
+    // The file is empty. Skip.
+    return true;
   }
 
   const formatted = prettier.check(text, {

prettier/main_test.ts

@@ -48,7 +48,11 @@ async function clearTestdataChanges(): Promise<void> {
 test(async function testPrettierCheckAndFormatFiles() {
   await clearTestdataChanges();
 
-  const files = [join(testdata, "0.ts"), join(testdata, "1.js")];
+  const files = [
+    join(testdata, "0.ts"),
+    join(testdata, "1.js"),
+    join(testdata, "2.ts")
+  ];
 
   var { code, stdout } = await run([...cmd, "--check", ...files]);
   assertEquals(code, 1);