refactor SimpleWorld to use Optional for storing accounts and extra null checks

[lu-pinto]

PR description

Fix several NPEs in SimpleWorld and I also decided to use Optional for accounts since the return from get/getAccount was ambiguous between an account that has been deleted or has never existed. Now there's a clear distinction between deleted (Optional.empty()) or has never existed (null). Also getTouchedAccounts returned null values when there were deleted accounts within which can be dangerous to the caller.

Note: SimpleWorld seems to only be used by the test framework, so this is not critical.

Fixed Issue(s)

fixes #7524

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