[Bug] SONiC Extension Packages Migration Error During New SONiC Image Install

* In 17/05/2023, SONiC updates the format of CLI section in the manifest.json.
* The implementation was updated in 202311 or later branches.
* However, the PR #2753 reported there is a error occur when the CLI migration.
* The root cause is occurred while a CLI field (show/config/clear) is empty ('').
* This PR modifies the parser of the cli_plugins field in the manifest.py.

Author: PeterTSW-EC

sonic_package_manager/manifest.py

@@ -88,7 +88,7 @@ def marshal(self, value):
                         raise ManifestError(f'{value} has items not of type {self.type.__name__}')
                 return value
             elif isinstance(value, self.type):
-                return [value]
+                return [] if not value else [value]
             else:
                 raise ManifestError(f'{value} is not of type {self.type.__name__}')