Only assign setters if present (#1405)

* Only assign setters if present

Rather than generating

```kotlin
val result: Content
result = Content()
result.content = if (contentSet) content else result.content
result.text = if (textSet) text else result.text
```

This will now conditionally set

```kotlin
val result: Content
result = Content()
if (contentSet) {
  result.content = content
}
if (textSet) {
  result.text = text
}
```

* Spotless

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/AdapterGenerator.kt

@@ -643,13 +643,14 @@ internal class AdapterGenerator(
         continue // Property already handled.
       }
       if (property.hasLocalIsPresentName) {
+        result.beginControlFlow("if (%N)", property.localIsPresentName)
         result.addStatement(
-          "%1N.%2N = if (%3N) %4N else %1N.%2N",
+          "%N.%N = %N",
           resultName,
           property.name,
-          property.localIsPresentName,
           property.localName
         )
+        result.endControlFlow()
       } else {
         result.addStatement(
           "%1N.%2N = %3N ?: %1N.%2N",