fix: drop word spacing in Japanese ("5 分 前" -> "5分前")

[tnayuki]

Japanese does not put spaces between words, but the Japanese language inherited the default " " for between_value_and_word / between_chunks / override_space_near_ago, so durations rendered as "5 分 前" and "1 週間 前" instead of "5分前" / "1週間前".

The Japanese impl dates back to #13 (2018), before those spacing hooks existed; when they were added (#36) with a " " default, Japanese was never updated to opt out. This overrides the three hooks to "", the same approach Korean already carries for between_value_and_word (#38).

Adds a test (Japanese had none) covering both the direct and the BoxedLanguage paths, mirroring the Korean test.