Spring Revise use of Objects.requireNonNull()

Historically, we have rarely intentionally thrown a NullPointerException in the Spring Framework.

Instead, we aim to throw either an IllegalArgumentException or IllegalStateException instead of a NullPointerException.

However, changes to the code in recent times have introduced the use of Objects.requireNonNull(Object) which throws a NullPointerException without an explicit error message.

The latter ends up providing less context than a NullPointerException thrown by the JVM (since Java 14) due to actually de-referencing a null-pointer. See https://openjdk.org/jeps/358.

In light of that, we have decided to revise our current use of Objects.requireNonNull(Object) by replacing it with Assert.notNull(), Assert.state(), Objects.requireNonNull(T, String), or Objects.requireNonNull(T, Supplier) as appropriate.

However, we will only use one of the Objects.requireNonNull(...) variants when we are required to throw a NullPointerException in order to comply with a third-party contract such as Reactive Streams.