fs: nullish coalescing to respect zero positional reads

When the file read position is moved passing zero is
not respected and `null` is used instead. PR fixes the
issues by using nullish coalescing which will return
the rhs only when the lhs is `null` or `undefined`;
respecting the zero.
Fixes: #40715
PR-URL: #40716Fixes: #40699
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>

Author: mihilmy

lib/internal/fs/promises.js

@@ -521,7 +521,7 @@ async function read(handle, bufferOrOptions, offset, length, position) {
 }
 offset=bufferOrOptions.offset||0;
 length=buffer.byteLength;
-position=bufferOrOptions.position||null;
+position=bufferOrOptions.position??null;
 }
 
 if(offset==null){

test/parallel/test-fs-promises-file-handle-read.js

@@ -68,6 +68,25 @@ async function validateReadNoParams() {
 awaitfileHandle.read();
 }
 
+// Validates that the zero position is respected after the position has been
+// moved. The test iterates over the xyz chars twice making sure that the values
+// are read from the correct position.
+asyncfunctionvalidateReadWithPositionZero(){
+constopts={useConf: true};
+constfilePath=fixtures.path('x.txt');
+constfileHandle=awaitopen(filePath,'r');
+constexpectedSequence=['x','y','z'];
+
+for(leti=0;i<expectedSequence.length*2;i++){
+constlen=1;
+constpos=i%3;
+constbuf=Buffer.alloc(len);
+const{ bytesRead }=awaitread(fileHandle,buf,0,len,pos,opts);
+assert.strictEqual(bytesRead,len);
+assert.strictEqual(buf.toString(),expectedSequence[pos]);
+}
+}
+
 
 (asyncfunction(){
 tmpdir.refresh();
@@ -78,4 +97,5 @@ async function validateReadNoParams() {
 awaitvalidateLargeRead({useConf: false});
 awaitvalidateLargeRead({useConf: true});
 awaitvalidateReadNoParams();
+awaitvalidateReadWithPositionZero();
 })().then(common.mustCall());