Fix generation of query parameters for optional/nullable function arguments

[ghost]

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Description of the PR

Fixes #3880

For a more complex example than the one from the issue, the generated code looks like this:

fun getClients(withHostname: kotlin.String, page: kotlin.Int?, size: kotlin.Int?, withOrg: kotlin.String?) : Clients {
    val localVariableBody: kotlin.Any? = null
    val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
        .apply {
            if (page != null) {
                put("page", listOf(page.toString()))
            }
            if (size != null) {
                put("size", listOf(size.toString()))
            }
            put("withHostname", listOf(withHostname.toString()))
            if (withOrg != null) {
                put("withOrg", listOf(withOrg.toString()))
            }
        }

    [...]
}

Reviewers: @jimschubert , @dr4ke616 , @karismann , @Zomzog

[wing328]

Tested locally with optional query parameters and the code looks good.

[wing328]

@atetzner thanks again for the PR. I've filed #4197 to better support nulalble for the query parameters

[wing328]

@atetzner thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569