Add EnforcedStyleForEmptyBraces parameter in SpaceAroundBlockBraces.

Make {} the default style.
But writing empty blocks as { } is common enough for us to make the style configurable.
Fixes #594.
Add param SpaceInsideHashLiteralBraces/EnforcedStyleForEmptyBraces.

And change EnforcedStyleIsWithSpaces to EnforcedStyle.
Merge pull request #598 from jonas054/fix_empty_braces_bug

Fix empty braces bug