kbuild: fix build error during make htmldocs

Fix the following build error when do 'make htmldocs':

 DOCPROC Documentation/DocBook/debugobjects.xml
exec /scripts/kernel-doc: No such file or directory
exec /scripts/kernel-doc: No such file or directory

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/basic/docproc.c

@@ -385,7 +385,7 @@ int main(int argc, char *argv[])
 	if (!srctree)
 		srctree = getcwd(NULL, 0);
 	kernsrctree = getenv("KBUILD_SRC");
-	if (!kernsrctree)
+	if (!kernsrctree || !*kernsrctree)
 		kernsrctree = srctree;
 	if (argc != 3) {
 		usage();