Fixes #197 where using filter=count resulted in empty objects being printed in list responses.

Problem

When filter=count is applied, CloudMonkey still iterates over list items and prints empty {} entries, even when the filtered field does not exist in those list objects.

Fix

• Skip empty rows after filtering.
• Omit list fields when no valid entries remain after applying the filter.
• Preserve only relevant top-level fields (for example, "count").

Result

Before, a command such as:

(localcloud) > list usagerecords startdate=2025-10-07 enddate=2025-10-10 type=6 usageid=2c218702-abff-46e8-bc8a-3e12fffa2188 filter=count

could return:

{
  "count": 319,
  "usagerecord": [
    {},
    {},
    ...
    {}
  ]
}

Now the output correctly returns only the top-level count field:

{
  "count": 319
}

instead of including empty objects.