fix: add nil pointer check for active cluster selection policy mappers (#7398)

<!-- Describe what has changed in this PR -->
**What changed?**
- add nil pointer check for active cluster selection policy mappers

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**Why?**
We want to remove those fields from ActiveClusterSelectionPolicy.
However, the change is not safe to rollback because of the nil-pointer
dereference of the new data that doesn't have those fields serialized
into thrift. This change is to create a commit that's safe to be rolled
back.

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test? Checked in staging env? -->
**How did you test it?**
unit tests

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**Potential risks**

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**Release notes**

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**Documentation Changes**

Author: Shaddoll

common/types/mapper/thrift/shared.go

@@ -7059,9 +7059,9 @@ func ToActiveClusterSelectionPolicy(t *shared.ActiveClusterSelectionPolicy) *typ
 	}
 	return &types.ActiveClusterSelectionPolicy{
 		ActiveClusterSelectionStrategy: ToActiveClusterSelectionStrategy(t.Strategy),
-		ExternalEntityType:             *t.ExternalEntityType,
-		ExternalEntityKey:              *t.ExternalEntityKey,
-		StickyRegion:                   *t.StickyRegion,
+		ExternalEntityType:             t.GetExternalEntityType(),
+		ExternalEntityKey:              t.GetExternalEntityKey(),
+		StickyRegion:                   t.GetStickyRegion(),
 		ClusterAttribute:               ToClusterAttribute(t.ClusterAttribute),
 	}
 }

common/types/mapper/thrift/shared_test.go

@@ -3591,6 +3591,8 @@ func TestQueueStateConversion(t *testing.T) {
 	}
 }
 
+// TODO: The way we're testing mappers doesn't have good coverage. The round trip tests don't cover serialization and deserialization of thrift.
+// We should also generate test data in thrift types and do round trip tests from thrift to internal and back to thrift.
 func TestActiveClusterSelectionPolicyConversion(t *testing.T) {
 	testCases := []*types.ActiveClusterSelectionPolicy{
 		nil,