fix Buffer.fill with a non-null empty fill including uninitialized bytes

src/bun.js/bindings/JSBuffer.cpp

@@ -601,6 +601,11 @@ static inline JSC::EncodedJSValue jsBufferConstructorFunction_allocBody(JSC::JSG
             }
             auto startPtr = uint8Array->typedVector() + start;
             auto str_ = value.toWTFString(lexicalGlobalObject);
+            if (str_.isEmpty()) {
+                memset(startPtr, 0, length);
+                RELEASE_AND_RETURN(scope, JSC::JSValue::encode(uint8Array));
+            }
+
             ZigString str = Zig::toZigString(str_);
 
             if (UNLIKELY(!Bun__Buffer_fill(&str, startPtr, end - start, encoding))) {

test/js/node/buffer.test.js

@@ -964,6 +964,10 @@ for (let withOverridenBufferWrite of [false, true]) {
         expect(buf[4]).toBe(128);
       });
 
+      it("fill(N, empty string) should be the same as fill(N) and not include any uninitialized bytes", () => {
+        expect(Buffer.alloc(100, "")).toEqual(Buffer.alloc(100));
+      });
+
       // https://github.com/joyent/node/issues/1758
       it("check for fractional length args, junk length args, etc.", () => {
         // Call .fill() first, stops valgrind warning about uninitialized memory reads.