fix(DBCluster): fix local write forwarding error on cluster update (c…

[kylenie-aws]

This is a continuation for PR #552 which to fix an issue causing resource updates to fail with a "Local Write Forwarding is already disabled/enabled on cluster" error.

This issue occurs because RDS validates the enableLocalWriteForwarding variable and will throw an exception if it is already disabled/enabled but still receives the same value in the modify request. PR #552 introduced a fix to set enableLocalWriteForwarding in the modify request only when this property is actually changing in the CFN template. However, there are two issues remaining:

1. Transforming the property from null -> false still encounters an 'already disabled' exception because CFN passes false to the modify request when the status is already false.
2. Transforming the property from true -> null fails to update the LocalWriteForwarding status to disabled. This occurs because CFN passes null to the modify request, which results in RDS not updating the status.

This fix did the following:

1. Explicitly set enableLocalWriteForwarding: null when the property is from null -> false, allows RDS to retain the status as false without throwing an exception."
2. Explicitly set enableLocalWriteForwarding: false when the property is from true -> null, forces RDS to update the status to false.

Below is the truth table after the fix:

PreviousCFNValue	DesiredCFNValue	ValueInModifyRequest	CurrentDBClusterState	DesiredDBClusterState
null	null	null	false	false
true	null	false	true	false
false	null	null	false	false
null	true	true	false	true
true	true	null	true	true
false	true	true	false	true
null	false	null	false	false
true	false	false	true	false
false	false	null	false	false

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