Emitter: Don't output trailing space for empty scalar nodes (#186)

See issue #46

Passing emitter tests:
* 2XXW: Spec Example 2.25. Unordered Sets
* 5WE3: Spec Example 8.17. Explicit Block Mapping Entries
* 6KGN: Anchor for empty node
* 6XDY: Two document start markers
* 7W2P: Block Mapping with Missing Values
* 8KB6: Multiline plain flow mapping key without value
* 9BXH: Multiline doublequoted flow mapping key without value
* C2DT: Spec Example 7.18. Flow Mapping Adjacent Values
* JTV5: Block Mapping with Multiline Scalars
* KK5P: Various combinations of explicit block mappings
* LE5A: Spec Example 7.24. Flow Nodes
* UT92: Spec Example 9.4. Explicit Documents
* W42U: Spec Example 8.15. Block Sequence Entry Types
* W4TN: Spec Example 9.5. Directives Documents
* ZWK4: Key with anchor after missing explicit mapping value