An anchor on an empty mapping value (`a: &anchor`) failed to register, so a
later `*anchor` errored as an undefined alias. Two bugs compounded:
- The parser dispatched the anchor-only value position to a nested block
mapping, absorbing the sibling key as the anchored node's child instead of
emitting the empty anchored node. It now emits an empty anchored node when
only a sibling mapping key follows, since the indented and inline-nested
cases are already handled by earlier branches.
- The stream composer registered an anchor by its raw scalar text, so an
anchor on an empty scalar came back as "" and a typed scalar (`&a 42`) as
text. It now registers the composed value, fixing a latent typed-anchor bug.
Fixes yaml-test-suite 6KGN; raises parse-level conformance to 306/333 (91.9%)
and value-level to 199/199 (100.0%), with the should-error rate unchanged.