Some decoders support decoding a proper payload from empty DataBuffers.
One good example is the ProtobufDecoder.

This patch adds a method canDecodeEmptyDataBuffer() to the Decoder
interface, allowing the Decoder to signal that it is able to do that.

In this case, the PayloadMethodArgumentResolver should pass the empty
DataBuffer to the Decoder, and in all other cases behave like before.

The `filterNonEmptyDataBuffer()` method was added via the fix for spring-projectsgh-26344. The comment states that this fix was done because for some Decoders, an empty input may not yield a valid message. Since this is now fixed by asking the decoder whether it can decode an empty DataBuffer, the test is now able to yield a value from empty input.

…eflect that its only meant for complete messages

… to better reflect the usecase