Fix USER_PATH resolution on case-insensitive filesystems (#25853)

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.

builddefs/build_keyboard.mk

@@ -474,8 +474,10 @@ ifneq ($(wildcard $(QMK_USERSPACE)),)
 endif
 
 # If the equivalent users directory exists in userspace, use that in preference to anything currently in the main repo
-ifneq ($(wildcard $(QMK_USERSPACE)/$(USER_PATH)),)
-    USER_PATH := $(QMK_USERSPACE)/$(USER_PATH)
+ifneq ($(QMK_USERSPACE),)
+	ifneq ($(wildcard $(QMK_USERSPACE)/$(USER_PATH)),)
+    	USER_PATH := $(QMK_USERSPACE)/$(USER_PATH)
+	endif
 endif
 
 # Pull in user level rules.mk