MB-55677 Fix the bug when parameter is None

Len function fails if passed a None value. Also the right test in
_post_form_encoded function is to check if the params is a string.
Here we're fixing this issue.

Change-Id: I603e4a82a53fe7cd2a71c81e478d82eefed7ec72
Reviewed-on: https://review.couchbase.org/c/couchbase-cli/+/187452
Well-Formed: Restriction Checker
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: James Lee <james.lee@couchbase.com>

Author: Amir Biglarbegian

cluster_manager.py

@@ -2399,7 +2399,7 @@ def _get(self, url, params=None):
     @request
     def _post_form_encoded(self, url, params):
         if self.debug:
-            if len(params) and not isinstance(params[0], tuple):
+            if params is not None and isinstance(params, str):
                 print(f'POST {url} {params}')
             else:
                 print(f'POST {url} {self._url_encode_params(params)}')