fix: only update item fields when flags are explicitly set

[natustx]

Summary

• Fix todoist modify overwriting all item fields with empty values when flags aren't provided
• Only update content, priority, labels, and due date when their respective flags are explicitly set
• Filter empty strings from label-names to prevent sending [""] to the API

Problem

Previously, running todoist modify abc123 -d tomorrow would:

• ✅ Set due date to "tomorrow"
• ❌ Clear content to ""
• ❌ Clear labels to empty
• ❌ Reset priority to 0

This happened because the code unconditionally overwrote all fields regardless of whether flags were provided.

Solution

Use c.IsSet() to check if each flag was explicitly provided before modifying the corresponding field.

Test plan

• Run todoist modify <id> -d tomorrow and verify only due date changes
• Run todoist modify <id> -c "new content" and verify only content changes
• Run todoist modify <id> -p 1 and verify only priority changes
• Run todoist modify <id> --label-names "label1,label2" and verify only labels change

[jenssegers]

Running into this issue as well!

[Trevortds]

I was expecting this might help my issue with the modify command (#287), but building from your fork the issue persists: the modify command does not work at all. Not in this branch, and not in the upstream.