Exclude blank answers from frequency lists

[Copilot]

The frequency list aggregation pipeline filters out null values but allows empty strings through, resulting in blank entries in the frequency tables.

Changes

• Updated the MongoDB aggregation filter in get-frequency-list.js to exclude both null and empty string values using $nin operator

// Before
{ $match: { _id: { $ne: null } } }

// After  
{ $match: { _id: { $nin: [null, ''] } } }

After the pipeline extracts answers via regex and trims whitespace, some answers become empty strings. These now get filtered out along with null values.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Exclude blank answers from frequency lists</issue_title>
<issue_description></issue_description>

<agent_instructions>do not write any tests, the user will test this manually</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Exclude blank answers from frequency lists #439

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