Support users to handle null types

# Problem solved by the feature

`class Hello {
@SerializedName("worlds")
@JvmField
val worlds = ArrayList()
@SerializedName("world")
@JvmField
val world = World()
@SerializedName("word")
@JvmField
var word = Word()
}
fun main() {
val hello = Gson().fromJson("{\"worlds\":null, \"world\":null, \"word\":null}", Hello::class.java)

println(hello.worlds) // null, this is not expected
println(hello.world) // null, this is not expected
println(hello.word) // null, ok
}
`
Since the field is final, I hope that when the value of "worlds" in json is null, no processing will be done.
When the value of "worlds" is not empty, the effect is like "worlds.add(xxx)".
Now the ArrayList instance is created and replaced by java.lang.reflect.Field.set(xxx,xxx).

# Feature description

excludeField cannot satisfy, because it's not based on parsing the scene.
ReflectiveExTypeAdapterFactory.java:162
`
if (customPolicy.shouldFieldSet(value, fieldValue, field, isPrimitive)) {
field.set(value, fieldValue);
}
`

# Alternatives / workarounds