Prevent construction of `null`-valued `JsonNode`s (like `TextNode`)

[cowtowncoder]

Describe your Issue

(note: follow-up to #4378 which is for 2.x)

We should not allow constructing JsonNodes around null, except for explicit NullNode: attempts to do so should throw a JacksonException (DatabindException subtype).
This should be checked for:

• TextNode
• BigIntegerNode
• DecimalNode
• BinaryNode

There may also be cases where we decide null is appropriate content.
One such type is POJONode.

[cowtowncoder]

cc @pjfanning .

[pjfanning]

I would include BinaryNode here too.

[cowtowncoder]

@pjfanning Ok I have no strong opinion so sure, why not.

EDIT: BinaryNode included as well.