fix(core): prevent spurious changes for unchanged ""/null in maps

- Use property presence checks (key in obj) in diffMap instead of
falsy checks to detect adds/deletes, avoiding treating ""/null as missing.
- Preserve strict equality/deepEqual path so unchanged "" and null emit no
changes.
- Add tests ensuring unchanged empty string and null fields produce zero
changes.
- No impact on list/movable/text/tree diff behavior.

Author: zxch3n

packages/core/src/core/diff.ts

@@ -998,8 +998,9 @@ export function diffMap<S extends ObjectLike>(
             childSchema?.getContainerType() ??
             tryInferContainerType(newItem, inferOptions);
 
-        // added new key
-        if (!oldItem) {
+        // Added new key: detect by property presence, not truthiness.
+        // Using `!oldItem` breaks for valid falsy values like "" or null.
+        if (!(key in oldStateObj)) {
             // Inserted a new container
             if (containerType) {
                 changes.push({

packages/core/tests/core/diff-equality.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { LoroDoc } from "loro-crdt";
+import { diffContainer } from "../../src/core/diff";
+import { schema } from "../../src/schema";
+
+describe("diffMap equality for falsy primitives", () => {
+    it("does not emit changes when string field stays ''", () => {
+        const doc = new LoroDoc();
+        // Ensure the container exists (name must match schema field)
+        doc.getMap("profile");
+
+        const rootSchema = schema({
+            profile: schema.LoroMap({
+                bio: schema.String(),
+            }),
+        });
+
+        const oldState = { profile: { bio: "" } } as const;
+        const newState = { profile: { bio: "" } } as const;
+
+        const changes = diffContainer(doc, oldState, newState, "", rootSchema);
+        expect(changes.length).toBe(0);
+    });
+
+    it("does not emit changes when field stays null", () => {
+        const doc = new LoroDoc();
+        doc.getMap("profile");
+
+        const rootSchema = schema({
+            profile: schema.LoroMap({
+                note: schema.String(),
+            }),
+        });
+
+        const oldState = { profile: { note: null } } as const;
+        const newState = { profile: { note: null } } as const;
+
+        const changes = diffContainer(doc, oldState, newState, "", rootSchema);
+        expect(changes.length).toBe(0);
+    });
+});
+