fix: avoid unnecessary read version increments (#15777)

Fixes #15262

Author: dummdidumm

.changeset/sweet-adults-complain.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid unnecessary read version increments

packages/svelte/src/internal/client/runtime.js

@@ -469,7 +469,7 @@ export function update_reaction(reaction) {
 		// we need to increment the read version to ensure that
 		// any dependencies in this reaction aren't marked with
 		// the same version
-		if (previous_reaction !== reaction) {
+		if (previous_reaction !== null && previous_reaction !== reaction) {
 			read_version++;
 
 			if (untracked_writes !== null) {