Make username optional for create secret git

Git authentication with basic auth doesn't necessarily require a
username to be present.

Signed-off-by: talife <gilletvincent@gmail.com>
Signed-off-by: Max Jonas Werner <mail@makk.es>

cmd/flux/testdata/create_secret/git/secret-git-only-pwd.yaml

@@ -0,0 +1,9 @@
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: podinfo-auth
+  namespace: my-namespace
+stringData:
+  password: my-password
+