Fix generation of query parameters for optional/nullable function arguments

Created by: ghost

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Fixes #3880 (closed)

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