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>

Author: mayankagarwals

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
+
+constcommon=require('../common');
+constassert=require('assert');
+constfs=require('fs');
+constfsPromises=fs.promises;
+constfixtures=require('../common/fixtures');
+constfilepath=fixtures.path('x.txt');
+
+constbuf=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(()=>{}));
+}));
+}));
+
+letfilehandle=null;
+
+// Test for promise api
+(async()=>{
+filehandle=awaitfsPromises.open(filepath,'r');
+constreadObject=awaitfilehandle.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)
+awaitfilehandle.close();
+});