…ns toClass() (#5201)

### Type of PR

Hello! While reviewing the Kotlin part of Armeria, I noticed that in the `FlowResponseConverterFunctionProvider` class, there's an extension function for `Type` used in the `createResponseConverterFunction()` function of `DelegatingResponseConverterFunctionProvider`. The `toClass()` function returns a type of `Class<*>?`.

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In the past, when working with Kotlin, I found it beneficial to define extension functions that return nullable values with names following the `toXXXOrNull()` pattern. This made the code more understandable for me and my teammates, especially during maintenance.

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Since the extension function is defined just below `createResponseConverterFunction()`, it may lead to questions like "Why use `toClass()?.let` here?" Adding `OrNull` to the function name can make the code more readable.

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> I understand that naming conventions can vary among teams, and it may not be a critical issue. However, if you find the suggestion helpful, I'd appreciate it if you could consider applying the change.

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```kt
private fun Type.toClassOrNull(): Class<*>? =
    when (this) {
        is ParameterizedType -> this.rawType as Class<*>
        is Class<*> -> this
        else -> null
    }