Fix empty query parameters in expand_params method

Previously, there was no distinction between parameters passed to the
backend with an empty value.

For example, when parsing the query string `?expand`, Rails would
include the key but with a nil value in the `params` hash. When this
hash was later serialized back into a query string using the `.to_query`
method, it erroneously resulted in `?expand=`.

This incorrect modification caused the backend to throw a 400 error.

This commit resolves the issue by splitting query parameters into two
distinct groups of parameters and handling them differently.

Author: eduardoj

src/api/app/lib/backend/connection_helper.rb

@@ -114,8 +114,13 @@ def expand_params(params, expand)
 
         expanded_params += params.delete(key).map { |value| value.to_query(key) } if params.key?(key)
       end
-      expanded_params += [params.to_query] unless params.empty?
-      expanded_params
+
+      empty_query_parameters = params.select { |_, v| v.nil? }
+      expanded_params += [empty_query_parameters.keys.join('&')]
+
+      expanded_params += [params.except(*empty_query_parameters.keys).to_query]
+
+      expanded_params.compact_blank
     end
 
     def calculate_endpoint(endpoint)