[Kotlin][Spring] optional types for required=false without defaultValue

Created by: markus-wa

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cc: @jimschubert (2017/09) @dr4ke616 (2018/08)

Description of the PR

This fixes a regression introduced (presumeably) by #1107 / #1106 (closed)

The parameter types should only be non-nullable if they are either required or have a default value. Since defaultValue is "null" (a string) by default, mustache treats it as a value (not-false or unset) the {{#defaultValue}}bla{{/defaultValue}} branch was alway hit, never {{^defaultValue}}.....

This change follows a similar pattern of the C# generator which uses the helper property isNullable for this instead.

It also fixes Failed to convert value of type 'java.lang.String' to required type 'java.lang.Integer'; nested exception is java.lang.NumberFormatException: For input string: "null" if the optional parameter isn't provided in the request by removing the defaultValue parameter if it's "null".