Fix properties being required and nullable:true (#16072)

* Make generated kotlin variable nullable if corresponding schema property is required and nullable

* Updating generated samples

* Fixing indent

* Updating samples after indent fix in mustache template

Author: fredrio

modules/openapi-generator/src/main/resources/kotlin-spring/dataClassReqVar.mustache

@@ -1,4 +1,4 @@
 {{#useBeanValidation}}{{>beanValidation}}{{>beanValidationModel}}{{/useBeanValidation}}{{#swagger2AnnotationLibrary}}
     @Schema({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeDoubleQuote}}{{{.}}}{{/lambdaEscapeDoubleQuote}}{{/lambdaRemoveLineBreak}}", {{/example}}required = true, {{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}description = "{{{description}}}"){{/swagger2AnnotationLibrary}}{{#swagger1AnnotationLibrary}}
     @ApiModelProperty({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeDoubleQuote}}{{{.}}}{{/lambdaEscapeDoubleQuote}}{{/lambdaRemoveLineBreak}}", {{/example}}required = true, {{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}value = "{{{description}}}"){{/swagger1AnnotationLibrary}}
-    @get:JsonProperty("{{{baseName}}}", required = true){{#isInherited}} override{{/isInherited}} {{>modelMutable}} {{{name}}}: {{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInCamelCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}
+    @get:JsonProperty("{{{baseName}}}", required = true){{#isInherited}} override{{/isInherited}} {{>modelMutable}} {{{name}}}: {{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInCamelCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{#isNullable}}?{{/isNullable}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}