Add Changelog entry

Author: schmidt-sebastian

packages/firestore/CHANGELOG.md

@@ -1,9 +1,11 @@
-# 0.7.3 (Unreleased)
+# 0.7.4 (Unreleased)
 - [fixed] Fixed an issue where the first `get()` call made after being offline
   could incorrectly return cached data without attempting to reach the backend.
 - [changed] Changed `get()` to only make 1 attempt to reach the backend before
   returning cached data, potentially reducing delays while offline. Previously
   it would make 2 attempts, to work around a backend bug.
+- [fixed] Fixed an issue that caused us to drop empty objects from a call to
+  `set(..., { merge: true })`.
 
 # 0.7.2
 - [fixed] Fixed a regression that prevented use of Firestore on ReactNative's