Fix missing null checks in DataProviderRegistrator

[Lignium]

Fixes a NullPointerException when trying to write read-only keys. For example, when trying to merge data from one block state to another, an exception occurs when trying to merge in my case Key.IS_PASSABLE.

Example code:

@Listener
public void onStartedServer(StartedEngineEvent<Server> event) {
    Objects.requireNonNull(event, "event");

    BlockState firstState = BlockTypes.OAK_LOG.get().defaultState().with(Keys.AXIS, Axis.Z).get();
    BlockState secondState = firstState.with(Keys.AXIS, Axis.Y).get();

    System.out.println(firstState);
    System.out.println(secondState);
    System.out.println(firstState.mergeWith(secondState)); // NPE
}

And exception:

java.lang.NullPointerException: null
	at org.spongepowered.common.data.provider.DataProviderRegistrator$ImmutableRegistrationBase$1.set(DataProviderRegistrator.java:501) ~[DataProviderRegistrator$ImmutableRegistrationBase$1.class:1.16.5-8.0.0-RC0]
	at org.spongepowered.common.data.provider.GenericImmutableDataProviderBase.with(GenericImmutableDataProviderBase.java:138) ~[GenericImmutableDataProviderBase.class:1.16.5-8.0.0-RC0]
	at org.spongepowered.common.data.holder.SpongeImmutableDataHolder.with(SpongeImmutableDataHolder.java:58) ~[SpongeImmutableDataHolder.class:1.16.5-8.0.0-RC0]
	at org.spongepowered.common.data.holder.SpongeImmutableDataHolder.mergeWith(SpongeImmutableDataHolder.java:72) ~[SpongeImmutableDataHolder.class:1.16.5-8.0.0-RC0]
	at org.spongepowered.api.data.DataHolder$Immutable.mergeWith(DataHolder.java:448) ~[DataHolder$Immutable.class:1.16.5-8.0.0-RC0]
	...

[dualspiral]

Is there any time where this is actually going to be null? I can't imagine there is a use case for a write-only key within Sponge.

[Lignium]

In fact, it is, but who knows, maybe someday it will be. Given that the get field is declared with @Nullable annotation, I think the check should be.