Fix NPE upon Git clone

The Change.getServerid() is flagged as @Nullable and therefore
could lead to a NPE when performing a permission check upon
Gerrit clone.

Bug: Issue 338826950
Release-Notes: Fix NPE during git clone
Change-Id: I7d9978ceb8397c9dde5a41fa92af93bce54bf1d9

java/com/google/gerrit/server/git/ChangesByProjectCacheImpl.java

diff --git a/java/com/google/gerrit/server/git/ChangesByProjectCacheImpl.java b/java/com/google/gerrit/server/git/ChangesByProjectCacheImpl.java
index b5a53b2..66c63c6 100644
--- a/java/com/google/gerrit/server/git/ChangesByProjectCacheImpl.java
+++ b/java/com/google/gerrit/server/git/ChangesByProjectCacheImpl.java
@@ -286,7 +286,7 @@
       int size = 0;
       size += JavaWeights.OBJECT; // change
       size += JavaWeights.REFERENCE + GerritWeights.KEY_INT; // changeId
-      size += JavaWeights.REFERENCE + c.getServerId().length();
+      size += JavaWeights.REFERENCE + (c.getServerId() == null ? 0 : c.getServerId().length());
       size += JavaWeights.REFERENCE + JavaWeights.OBJECT + 40; // changeKey;
       size += JavaWeights.REFERENCE + GerritWeights.TIMESTAMP; // createdOn;
       size += JavaWeights.REFERENCE + GerritWeights.TIMESTAMP; // lastUpdatedOn;