When QMK_USERSPACE is empty, the wildcard check in build_keyboard.mk
incorrectly matches user home directories on case-insensitive filesystems
(macOS/Windows), causing USER_PATH to resolve to an absolute path instead
of a relative path.

This only affects users whose username matches their user directory name
(e.g., username 'psycher' with home '/Users/psycher' and QMK user dir
'users/psycher').

Fix by checking if QMK_USERSPACE is non-empty before performing the
wildcard check.

Co-authored-by: Joel Challis <git@zvecr.com>

replace check + `include` with `-include`