fix: don't clear list when CSV import column prompt is cancelled

[Chessing234]

Resolves

Resolves #7313

Proposed Changes

When importing a multi-column .csv/.tsv into a list, Monitor's handleImport asks which column to use with window.prompt. The return value was passed straight into parseInt(...):

columnNumber = parseInt(prompt(msg), 10);

If the user presses Cancel, prompt returns null, so parseInt(null, 10) is NaN. Every row[NaN - 1] is then undefined, the .filter(item => typeof item === 'string') drops them all, and the list is overwritten with an empty array — silently deleting the list's existing contents.

This change reads the prompt response first and returns early when it is null, leaving the list untouched on cancel. Entering a valid column still works exactly as before.

Reason for Changes

Pressing Cancel should abort the import without changing the project, as described in the issue. The old code treated a cancelled prompt as "import an empty column," which wiped the list (High Severity data loss).

Test Coverage

Added test/unit/containers/monitor.test.jsx with two cases:

• cancelling the column prompt (prompt → null) does not call setVariableValue, so the list is unchanged;
• choosing a column (prompt → '2') imports that column (['a2', 'b2']).

The first test fails on the current master and passes with this change; the second guards against regressing normal imports. npm run test:unit -- test/unit/containers/monitor.test.jsx passes and eslint . --ext .js,.jsx is clean.

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