making LambdaArgument methods nullable

overridden `LambdaArgument.getArgumentExpression` removed because it is already nullable in parent

#EA-117013

plugins/uast-kotlin/src/org/jetbrains/uast/kotlin/KotlinUastLanguagePlugin.kt

@@ -328,7 +328,7 @@ internal object KotlinConverter {
                 }
 
             is KtExpression -> KotlinConverter.convertExpression(element, givenParent, requiredType)
-            is KtLambdaArgument -> KotlinConverter.convertExpression(element.getLambdaExpression(), givenParent, requiredType)
+            is KtLambdaArgument -> element.getLambdaExpression()?.let { KotlinConverter.convertExpression(it, givenParent, requiredType) }
             is KtLightAnnotationForSourceEntry.LightExpressionValue<*> -> {
                 val expression = element.originalExpression
                 when (expression) {