Dont trigger for non-optional type casting, or for optional types

[mildm8nnered]

Addresses #5802

We no longer trigger unless the type casting is optional (as?), or if the type is optional.

Essentially, in either case, the correction would change the resulting true or false value. See #5802 for details.

We also now check for nil != foo as? Bar cases

[SwiftLintBot]

	1 Warning
⚠️	This PR introduced a violation in Realm: /Realm/ObjectServerTests/ClientResetTests.swift:284:36: warning: Prefer Type Checking Violation: Prefer a is X to a as? X != nil (prefer_type_checking)

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