PLU-624: fix(formsg-frontend): table field preview

[kevinkim-ogp]

Problem

• Forms that contain Section or Image fields cannot preview their table rows due to a mismatch in the Zod schema.
  The current schema allows undefined, but these field types return null, which causes validation errors.
• If there is more than 1 table in the form, the preview always shows data from the first table

Solution

• Replace .optional() with .nullish() so that both undefined and null are accepted.
• Fetch the table data using the variable id

Tests

• All FormSG test results display correctly
• FormSG tables can be previewed correctly even when they contain section headers or images
• FormSG table previews display the correct data when there are two or more tables

[linear]

PLU-624 Investigate: zod error

[pregnantboy]

thanks for quick fix 🙏🏻