test: add test for fs.read when offset key is null

added test for uncovered if statement in lib/fs.js

PR-URL: #35918
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

test/parallel/test-fs-read-offset-null.js

@@ -0,0 +1,43 @@
+'use strict';
+
+
+// Test to assert the desired functioning of fs.read
+// when {offset:null} is passed as options parameter
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const fsPromises = fs.promises;
+const fixtures = require('../common/fixtures');
+const filepath = fixtures.path('x.txt');
+
+const buf = Buffer.alloc(1);
+// Reading only one character, hence buffer of one byte is enough
+
+// Test for callback api
+fs.open(filepath, 'r', common.mustSucceed((fd) => {
+  fs.read(fd, { offset: null, buffer: buf },
+          common.mustSucceed((bytesRead, buffer) => {
+            assert.strictEqual(buffer[0], 120);
+            // Test is done by making sure the first letter in buffer is
+            // same as first letter in file.
+            // 66 is the hex for ascii code of letter B
+
+            fs.close(fd, common.mustSucceed(() => {}));
+          }));
+}));
+
+let filehandle = null;
+
+// Test for promise api
+(async () => {
+  filehandle = await fsPromises.open(filepath, 'r');
+  const readObject = await filehandle.read(buf, null, buf.length);
+  assert.strictEqual(readObject.buffer[0], 120);
+})()
+.then(common.mustCall())
+.finally(async () => {
+// Close the file handle if it is opened
+  if (filehandle)
+    await filehandle.close();
+});