pkg/yaml: preserve implicit null in flow mappings

[s-johri]

Fixes #320

An empty scalar gets flow_plain_allowed = false, so in flow style the emitter
falls back to single quoting and an implicit null becomes ''. This writes the
null explicitly instead, so a: { b: } formats to a: {b: null}. Block style is
untouched.

I used the explicit null spelling because a bare empty scalar is only safe as a
flow mapping value, and the parser rejects a: [x, , y] and a: { : 1 }. Happy
to change it if you would rather keep the original bytes.

Added a round trip test in pkg/yaml/node_test.go that decodes a document to a
Node, encodes it again, and checks the data survives.