Templates/AutoMockable.stencil: fix stencil to consider nullable closures as escaping

[alexdmotoc]

Refines work done in #1126 by allowing generating invocation vars for methods with nullable closures, as those are implicitly escaping, for which it's safe to generate such vars

e.g.

Before

class NullableClosureProtocolMock: NullableClosureProtocol {
    var setClosureClosureVoidVoidCallsCount = 0
    var setClosureClosureVoidVoidCalled: Bool {
        return setClosureClosureVoidVoidCallsCount > 0
    }
    var setClosureClosureVoidVoidClosure: (((() -> Void)?) -> Void)?

    func setClosure(_ closure: (() -> Void)?) {
        setClosureClosureVoidVoidCallsCount += 1
        setClosureClosureVoidVoidClosure?(closure)
    }
}

After

class NullableClosureProtocolMock: NullableClosureProtocol {
    var setClosureClosureVoidVoidCallsCount = 0
    var setClosureClosureVoidVoidCalled: Bool {
        return setClosureClosureVoidVoidCallsCount > 0
    }
    var setClosureClosureVoidVoidReceivedClosure: (((() -> Void)))? // this was missing
    var setClosureClosureVoidVoidReceivedInvocations: [(((() -> Void)))?] = [] // this was missing
    var setClosureClosureVoidVoidClosure: (((() -> Void)?) -> Void)?

    func setClosure(_ closure: (() -> Void)?) {
        setClosureClosureVoidVoidCallsCount += 1
        setClosureClosureVoidVoidReceivedClosure = closure
        setClosureClosureVoidVoidReceivedInvocations.append(closure)
        setClosureClosureVoidVoidClosure?(closure)
    }
}