portageq get_repo_path: output empty if unknown

It used to output "None" for unknown repos, which was not intended.
diff --git a/bin/portageq b/bin/portageq
index a13ca99..f0ecbfc 100755
--- a/bin/portageq
+++ b/bin/portageq
@@ -627,7 +627,10 @@
 		print("ERROR: insufficient parameters!")
 		sys.exit(2)
 	for arg in argv[1:]:
-		print(portage.db[argv[0]]["porttree"].dbapi.getRepositoryPath(arg))
+		path = portage.db[argv[0]]["porttree"].dbapi.getRepositoryPath(arg)
+		if path is None:
+			path = ""
+		print(path)
 
 def list_preserved_libs(argv):
 	"""<root>