fix: unexpected cache hit when modifiedFiles size 0 (#47)

Fix unexpected cache hit in CI when modifiedFiles size 0 (matched pre
test case cache).

BTW, I'm curious why `postcssProcessorCache` is set as a static method.
`postcssProcessorCache` uses `entryName` as the cache key, which makes
the cache not strict when running rspack multiple times & there is an
import cache.

<img width="1152" alt="image"
src="https://github.com/user-attachments/assets/693f9f46-58af-435a-bffc-2b2afc33c57b"
/>


![image](https://github.com/user-attachments/assets/1d302c3d-ae74-4000-ad67-735b80739a31)



https://github.com/rspack-contrib/rsbuild-ecosystem-ci/actions/runs/14164766016/job/39675948751

Author: 9aoy

package.json

@@ -31,9 +31,9 @@
     "@biomejs/biome": "^1.9.4",
     "@playwright/test": "^1.50.1",
     "@rollup/pluginutils": "^5.1.4",
-    "@rsbuild/core": "^1.3.0-beta.0",
+    "@rsbuild/core": "^1.3.0",
     "@rslib/core": "^0.5.2",
-    "@rsbuild/webpack": "^1.2.3",
+    "@rsbuild/webpack": "^1.3.0",
     "@types/node": "^22.13.7",
     "@types/semver": "^7.5.8",
     "playwright": "^1.50.1",