Document-End marker should always be emitted after block scalar with trailing empty lines

The following YAML demonstrates a case where a document-end marker is important and should not be omitted:
```
foo: |+
  foo

...
```

libyaml should emit it even if `implicit` is true. Currently it does not, pyyaml and ruamel do.

Demo:
```
libyaml % echo '+STR
+DOC
=VAL |foo\n\n\n
-DOC
-STR' | ./tests/run-emitter-test-suite
|+
  foo


libyaml %
```