merge: #10491

10491: Do not use null as KeyGenerator r=saig0 a=Zelldon

## Description

Based on review hint here #10440 (comment) 
<!-- Please explain the changes you made here. -->

## Related issues

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closes #



Co-authored-by: Christopher Zell <zelldon91@googlemail.com>

engine/src/main/java/io/camunda/zeebe/engine/state/ZeebeDbState.java

@@ -45,6 +45,7 @@
 import io.camunda.zeebe.engine.state.mutable.MutableZeebeState;
 import io.camunda.zeebe.engine.state.processing.DbBlackListState;
 import io.camunda.zeebe.engine.state.variable.DbVariableState;
+import java.util.Objects;
 import java.util.function.BiConsumer;
 
 public class ZeebeDbState implements MutableZeebeState {
@@ -78,7 +79,7 @@ public ZeebeDbState(
       final KeyGenerator keyGenerator) {
     this.partitionId = partitionId;
     this.zeebeDb = zeebeDb;
-    this.keyGenerator = keyGenerator;
+    this.keyGenerator = Objects.requireNonNull(keyGenerator);
 
     variableState = new DbVariableState(zeebeDb, transactionContext);
     processState = new DbProcessState(zeebeDb, transactionContext);

engine/src/main/java/io/camunda/zeebe/engine/state/query/StateQueryService.java

@@ -65,9 +65,15 @@ private void ensureServiceIsOpened() {
     }
     if (state == null) {
       // service is used for the first time, create state now
-      // we don't need a key generator here, so we set it to null
+      // we don't need a key generator here, so we set it to unsupported
       state =
-          new ZeebeDbState(Protocol.DEPLOYMENT_PARTITION, zeebeDb, zeebeDb.createContext(), null);
+          new ZeebeDbState(
+              Protocol.DEPLOYMENT_PARTITION,
+              zeebeDb,
+              zeebeDb.createContext(),
+              () -> {
+                throw new UnsupportedOperationException("Not allowed to generate a new key");
+              });
     }
   }
 }