vsctl event get failed if no source is defined for event

[hwjiangkai]

Bug Report

vsctl event get failed if no source is defined for event，the error is reported as follows:
+---------------------------------------------------------------+
| ERROR |
+---------------------------------------------------------------+
| unmarshal http response data failed: [cannot parse uri ref: ] |
+---------------------------------------------------------------+

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your Vanus version? (Required)