Fixes 5912
When `control_brace_style = "AlwaysNextLine"`, the code seems to always assume that `body_prefix` is `{`. This is however not the case when `match_arm_blocks = false`. This causes `block_sep` to introduce extra white space that causes the error.
The fix was to check if `body_prefix` is empty before matching on `ControlBraceStyle::AlwaysNextLine`.
